feat: the artist picker on Patreon and SubscribeStar too — and the Patreon name is canon (milestone 429)

Operator: "yes add the artist picker to patreon and subscribestar too. but
generally we treat the patreon name as the canon"

- Every add now goes through the panel. On Patreon/SubscribeStar it opens
  on the creator's display name (read only when the panel opens:
  probe?names=1), searches FC's artists with it, and auto-picks a match.
  An untouched URL handle sends no name, so the server still resolves it.
- Patreon is canon: joining a Patreon source to an artist known by another
  name offers "Rename “x” to the Patreon name “X”", ticked by default.
  quick-add's use_platform_name renames server-side from the name it reads
  itself; name only, the slug never moves (#130); never to a URL handle when
  the name can't be read; ignored on SubscribeStar and Discord.
- _platform_display_name returns None rather than the handle, bounded by the
  same 6s lookup budget as Discord's names.
- panelDefaults / addRequest / renameOffer replace the Discord-only helpers.

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

extension/lib/chip.js

@@ -1,7 +1,7 @@
 /**
  * The content script's decisions, kept apart from its DOM so the specs can
  * load them (test/chip.spec.js): which state the chip shows, what it says,
- * and what the Discord Add panel starts out proposing.
+ * and what the Add panel starts out proposing and finally sends.
  *
  * `probe` is /api/extension/probe's answer; `platformName` is the display
  * name (PLATFORMS[key].name), passed in so this file needs no other lib.
@@ -46,37 +46,76 @@ function serverLabel(d) {
 }
 
 /**
- * What the Discord Add panel opens with. The channel is the default scope
- * when there is one: a server source walks every channel the token can read,
- * which is rarely what a single art channel wants. The artist is the probe's
- * suggestion (the owner of another source on this server), else a new artist
- * named after the server.
+ * What the Add panel opens with, on any platform.
+ *
+ * Discord: the channel is the default scope when there is one — a server
+ * source walks every channel the token can read, which is rarely what a
+ * single art channel wants. The artist is the probe's suggestion (the owner
+ * of another source on this server), else a new one named after the server.
+ *
+ * Patreon / SubscribeStar: the page URL is the source. The artist is the one
+ * whose slug the URL already names (artist_match), else a new one under the
+ * creator's display name — `probe.display_name`, from the probe the panel
+ * makes with names=1 — falling back to the URL handle, which the server
+ * resolves on its own when it is left untouched (`nameIsHandle`).
  */
-function discordPanelDefaults(probe) {
+function panelDefaults(probe, pageUrl) {
   const d = probe?.discord || {};
   const suggested = probe?.state === 'artist_match' && probe.artist ? probe.artist : null;
+  const discord = probe?.platform === 'discord';
+  const shown = probe?.display_name || null;
+  let artistName = '';
+  if (suggested) artistName = suggested.name;
+  else if (discord) artistName = d.server_name || '';
+  else artistName = shown || probe?.slug || '';
   return {
-    scope: d.channel_id ? 'channel' : 'server',
+    platform: probe?.platform || null,
+    scope: discord ? (d.channel_id ? 'channel' : 'server') : 'page',
+    pageUrl: pageUrl || null,
     channelUrl: d.channel_url || null,
     serverUrl: d.server_url || null,
     artist: suggested ? { id: suggested.id, name: suggested.name } : null,
-    artistName: suggested ? suggested.name : (d.server_name || ''),
+    artistName,
+    nameIsHandle: !discord && !suggested && !shown,
+    handle: probe?.slug || '',
+    displayName: shown,
+    // Patreon is canon: joining an artist known by another name takes the
+    // Patreon name, unless the operator unticks it.
+    adoptPlatformName: true,
   };
 }
 
+/**
+ * The rename the panel offers, or null: only on Patreon (the canon name),
+ * only when joining an existing artist, only with a name actually read from
+ * Patreon, and only when it differs from what the artist is called now.
+ */
+function renameOffer(choice) {
+  if (choice.platform !== 'patreon' || !choice.artist || !choice.displayName) return null;
+  if (choice.artist.name === choice.displayName) return null;
+  return { from: choice.artist.name, to: choice.displayName };
+}
+
 /**
  * The quick-add body for the panel's current choice. A picked artist goes by
  * id — names can collide once slugified — and a typed name creates (or
- * finds) that artist. null when there is nothing valid to send.
+ * finds) that artist. The URL handle left untouched sends no name, so the
+ * server resolves the display name itself. null when there is nothing valid.
  */
-function discordAddRequest(choice) {
-  const url = choice.scope === 'server' ? choice.serverUrl : choice.channelUrl;
+function addRequest(choice) {
+  let url = choice.pageUrl;
+  if (choice.scope === 'server') url = choice.serverUrl;
+  else if (choice.scope === 'channel') url = choice.channelUrl;
   if (!url) return null;
   if (choice.artist && choice.artist.id != null && choice.artist.name === choice.artistName) {
-    return { url, artistId: choice.artist.id };
+    const req = { url, artistId: choice.artist.id };
+    if (choice.adoptPlatformName && renameOffer(choice)) req.usePlatformName = true;
+    return req;
   }
   const name = (choice.artistName || '').trim();
-  return name ? { url, artistName: name } : null;
+  if (!name) return null;
+  if (choice.nameIsHandle && name === choice.handle) return { url };
+  return { url, artistName: name };
 }
 
 /**