feat: the extension adds Discord channels to an artist you pick, and its tests gate the XPI (milestone 429)

Server (#4420)
- extension_service gains a Discord pattern (server or channel, jump links,
  ptb/canary; not DMs or threads), mirrored in platforms.js and pinned by
  the shared artist-url-samples.json.
- probe on a Discord URL matches the source by ids under any artist, reports
  a whole-server source as covering the channel, suggests the artist who owns
  another source on the same server, and names server/channel via the stored
  token (best-effort, bounded, no rate-limit waits).
- quick-add takes artist_id / artist_name; Discord URLs are stored canonical.

Extension (#4421, #4422)
- Content script on discord.com; SPA navigation by URL polling (the old
  pushState patch ran in the isolated world and never fired); stale probes
  are dropped.
- Discord chip opens an Add panel: this channel or the whole server, and the
  suggested artist / a search / a new name.
- Popup: sources show artist, platform and state; a Discord token export is
  verified by FC and the result shown. Token capture covers ptb/canary.
- Pure logic in lib/chip.js and lib/popup-format.js, with specs.

CI (#4423)
- extension.yml's lane (web-ext lint, vitest, XPI contents) moves into
  build.yml as extension-test and joins the needs of sign-extension,
  build-web and build-agent. As a separate workflow it gated nothing: a red
  extension suite still signed and shipped the XPI (rule 177).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

extension/content/content-script.js

@@ -5,42 +5,61 @@
   // Cached probe result for the current URL so click-handlers know which
   // action to dispatch without round-tripping again.
   let currentProbe = null;
+  // Bumped on every evaluate(): a probe that answers after the operator has
+  // navigated on is for a page they've left, and must not repaint the chip.
+  let generation = 0;
+  let lastUrl = window.location.href;
 
   evaluate();
 
-  const reEval = () => evaluate();
-  window.addEventListener('popstate', reEval);
-  const origPush = history.pushState;
-  history.pushState = function () { origPush.apply(this, arguments); reEval(); };
+  // SPA navigation. Patreon, SubscribeStar and above all Discord change
+  // channel/page without a reload. Patching history.pushState from here never
+  // worked: a content script runs in an isolated world, so the page's own
+  // pushState is not the function we'd replace. Polling the URL is the one
+  // signal that sees every navigation, and costs a string compare.
+  window.addEventListener('popstate', () => onUrlMaybeChanged());
+  setInterval(onUrlMaybeChanged, 500);
+
+  function onUrlMaybeChanged() {
+    if (window.location.href === lastUrl) return;
+    lastUrl = window.location.href;
+    closePanel();
+    evaluate();
+  }
 
   async function evaluate() {
+    const mine = ++generation;
     const url = window.location.href;
     const platform = getPlatformFromUrl(url);
     const onArtist = platform && isArtistPage(url, platform);
-    const btn = document.getElementById('fc-add-source-btn');
     if (!onArtist) {
-      if (btn) btn.remove();
+      removeButton();
       currentProbe = null;
       return;
     }
-    // On artist pages, ask the backend what state the URL is in BEFORE
-    // injecting the button — so the chip can render the right state on
-    // first paint instead of flashing the generic "Add" copy and
-    // updating afterwards.
+    // Ask the backend what state the URL is in BEFORE drawing the button, so
+    // the chip renders the right state on first paint instead of flashing the
+    // generic "Add" copy and updating afterwards.
     let probe;
     try {
       probe = await browser.runtime.sendMessage({ type: 'PROBE_SOURCE', url });
     } catch (e) {
       probe = { error: e?.message || 'probe failed' };
     }
+    if (mine !== generation) return;
     currentProbe = probe;
     if (probe?.state === 'unknown_platform') {
-      if (btn) btn.remove();
+      removeButton();
       return;
     }
     renderButton(probe);
   }
 
+  function removeButton() {
+    document.getElementById('fc-add-source-btn')?.remove();
+    closePanel();
+  }
+
   function renderButton(probe) {
     let btn = document.getElementById('fc-add-source-btn');
     if (!btn) {
@@ -51,79 +70,36 @@
     }
     // Reset state classes so re-renders (SPA navigation) don't stack.
     btn.className = 'fc-add-source-btn';
-    btn.classList.add(`fc-add-source-btn--${stateModifier(probe)}`);
-    btn.textContent = labelFor(probe);
+    btn.classList.add(`fc-add-source-btn--${chipState(probe)}`);
+    btn.textContent = chipLabel(probe, PLATFORMS[probe?.platform]?.name || probe?.platform || '');
     btn.disabled = false;
   }
 
-  function stateModifier(probe) {
-    if (!probe || probe.error) return 'new';
-    return ({
-      source_match: 'source-match',
-      artist_match: 'artist-match',
-      new: 'new',
-    })[probe.state] || 'new';
-  }
-
-  function labelFor(probe) {
-    if (!probe || probe.error) return '+ Add to FabledCurator';
-    const platformName = platformDisplayName(probe.platform);
-    const artistName = probe.artist?.name;
-    switch (probe.state) {
-      case 'source_match':
-        return `✓ In FabledCurator · ${platformName}`;
-      case 'artist_match':
-        return `+ Add ${platformName} source to ${artistName || 'artist'}`;
-      case 'new':
-      default:
-        return '+ Add to FabledCurator';
-    }
-  }
-
-  function platformDisplayName(key) {
-    return PLATFORMS[key]?.name || key || '';
-  }
-
   async function onClick() {
     const btn = document.getElementById('fc-add-source-btn');
     if (!btn) return;
-    btn.disabled = true;
-    const original = btn.textContent;
     const probe = currentProbe;
 
     if (probe?.state === 'source_match') {
-      btn.textContent = 'Opening…';
-      try {
-        const r = await browser.runtime.sendMessage({
-          type: 'OPEN_ARTIST_PAGE',
-          slug: probe.artist?.slug,
-        });
-        if (r?.error) showToast(`Error: ${r.error}`, 'error');
-      } catch (e) {
-        showToast(`Error: ${e.message}`, 'error');
-      } finally {
-        btn.disabled = false;
-        btn.textContent = original;
-      }
+      await openArtist(btn, probe.artist?.slug);
       return;
     }
+    // A Discord URL names a channel, not a creator — ask which artist.
+    if (probe?.platform === 'discord') {
+      if (document.getElementById('fc-discord-panel')) closePanel();
+      else openDiscordPanel(probe);
+      return;
+    }
+    await add(btn, { url: window.location.href });
+  }
 
-    btn.textContent = 'Adding…';
+  async function openArtist(btn, slug) {
+    btn.disabled = true;
+    const original = btn.textContent;
+    btn.textContent = 'Opening…';
     try {
-      const r = await browser.runtime.sendMessage({
-        type: 'ADD_AS_SOURCE',
-        url: window.location.href,
-      });
-      if (r?.error) {
-        showToast(`Error: ${r.error}`, 'error');
-      } else {
-        const verb = r.created_source ? 'Added' : 'Already a source for';
-        showToast(`${verb} ${r.artist?.name || 'artist'} (${r.source?.platform || ''})`, 'success');
-        // Re-probe so the chip flips green without waiting for the next
-        // navigation.
-        evaluate();
-        return;
-      }
+      const r = await browser.runtime.sendMessage({ type: 'OPEN_ARTIST_PAGE', slug });
+      if (r?.error) showToast(`Error: ${r.error}`, 'error');
     } catch (e) {
       showToast(`Error: ${e.message}`, 'error');
     } finally {
@@ -132,6 +108,153 @@
     }
   }
 
+  // One add, shared by the one-click chip and the Discord panel. Resolves
+  // true on success.
+  async function add(btn, request) {
+    btn.disabled = true;
+    const original = btn.textContent;
+    btn.textContent = 'Adding…';
+    try {
+      const r = await browser.runtime.sendMessage({ type: 'ADD_AS_SOURCE', ...request });
+      if (r?.error) {
+        showToast(`Error: ${r.error}`, 'error');
+        return false;
+      }
+      const verb = r.created_source ? 'Added to' : 'Already a source for';
+      showToast(`${verb} ${r.artist?.name || 'artist'} (${r.source?.platform || ''})`, 'success');
+      // Re-probe so the chip flips green without waiting for a navigation.
+      evaluate();
+      return true;
+    } catch (e) {
+      showToast(`Error: ${e.message}`, 'error');
+      return false;
+    } finally {
+      btn.disabled = false;
+      btn.textContent = original;
+    }
+  }
+
+  // ---- Discord Add panel ----
+  // Where: this channel or the whole server. Who: the suggested artist, one
+  // found by search, or a new one by name. Built with createElement only —
+  // server, channel and artist names are other people's text.
+
+  function el(tag, props = {}, children = []) {
+    const node = document.createElement(tag);
+    const { class: className, text, ...rest } = props;
+    if (className) node.className = className;
+    if (text != null) node.textContent = text;
+    Object.assign(node, rest);
+    for (const c of children) node.appendChild(c);
+    return node;
+  }
+
+  function closePanel() {
+    document.getElementById('fc-discord-panel')?.remove();
+  }
+
+  function openDiscordPanel(probe) {
+    closePanel();
+    const d = probe.discord || {};
+    const choice = discordPanelDefaults(probe);
+    let searchSeq = 0;
+
+    const scopeRow = (value, label, disabled) => {
+      const input = el('input', {
+        type: 'radio', name: 'fc-discord-scope', value,
+        checked: choice.scope === value, disabled,
+      });
+      input.addEventListener('change', () => { choice.scope = value; refresh(); });
+      return el('label', { class: 'fc-panel__radio' }, [input, el('span', { text: label })]);
+    };
+
+    const nameInput = el('input', {
+      type: 'text', class: 'fc-panel__input', value: choice.artistName,
+      placeholder: 'Artist name — search or type a new one',
+      autocomplete: 'off', spellcheck: false,
+    });
+    const results = el('div', { class: 'fc-panel__results' });
+    const hint = el('div', { class: 'fc-panel__hint' });
+    const addBtn = el('button', { class: 'fc-panel__btn fc-panel__btn--primary', text: 'Add' });
+    const cancelBtn = el('button', { class: 'fc-panel__btn', text: 'Cancel' });
+
+    const panel = el('div', { id: 'fc-discord-panel', class: 'fc-panel' }, [
+      el('div', { class: 'fc-panel__title', text: 'Add Discord source' }),
+      el('div', { class: 'fc-panel__sub', text: serverLabel(d) }),
+      el('div', { class: 'fc-panel__label', text: 'Follow' }),
+      scopeRow('channel', d.channel_id ? channelLabel(d) : 'this channel', !d.channel_id),
+      scopeRow('server', `Every channel in ${serverLabel(d)}`, false),
+      el('div', { class: 'fc-panel__label', text: 'Artist' }),
+      nameInput,
+      results,
+      hint,
+      el('div', { class: 'fc-panel__actions' }, [cancelBtn, addBtn]),
+    ]);
+
+    function refresh() {
+      const req = discordAddRequest(choice);
+      addBtn.disabled = !req;
+      if (!req) hint.textContent = 'Pick an artist or type a name.';
+      else if (req.artistId != null) hint.textContent = `Connects to ${choice.artist.name} in FabledCurator.`;
+      else hint.textContent = `Adds to “${req.artistName}” — created if FabledCurator has no artist by that name.`;
+    }
+
+    function showResults(rows) {
+      results.replaceChildren(...rows.map((a) => {
+        const row = el('button', { class: 'fc-panel__result', text: a.name });
+        row.addEventListener('click', () => {
+          choice.artist = { id: a.id, name: a.name };
+          choice.artistName = a.name;
+          nameInput.value = a.name;
+          results.replaceChildren();
+          refresh();
+        });
+        return row;
+      }));
+    }
+
+    let debounce = null;
+    nameInput.addEventListener('input', () => {
+      choice.artistName = nameInput.value;
+      refresh();
+      clearTimeout(debounce);
+      const q = nameInput.value.trim();
+      if (!q) { results.replaceChildren(); return; }
+      debounce = setTimeout(async () => {
+        const mine = ++searchSeq;
+        let r;
+        try {
+          r = await browser.runtime.sendMessage({ type: 'SEARCH_ARTISTS', q });
+        } catch {
+          return;
+        }
+        if (mine !== searchSeq || r?.error) return;
+        showResults(r.artists || []);
+      }, 200);
+    });
+    // Keep Discord's global shortcuts from eating keystrokes meant for us.
+    panel.addEventListener('keydown', (e) => {
+      e.stopPropagation();
+      if (e.key === 'Escape') closePanel();
+      if (e.key === 'Enter' && e.target === nameInput && !addBtn.disabled) addBtn.click();
+    });
+
+    cancelBtn.addEventListener('click', closePanel);
+    addBtn.addEventListener('click', async () => {
+      const req = discordAddRequest(choice);
+      if (!req) return;
+      const btn = document.getElementById('fc-add-source-btn');
+      addBtn.disabled = true;
+      const ok = await add(btn || addBtn, req);
+      if (ok) closePanel();
+      else refresh();
+    });
+
+    document.body.appendChild(panel);
+    refresh();
+    nameInput.focus();
+  }
+
   function showToast(text, kind) {
     const t = document.createElement('div');
     t.className = `fc-toast fc-toast--${kind}`;