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/lib/platforms.js

@@ -57,7 +57,8 @@ const PLATFORMS = {
     domains: ['.discord.com', 'discord.com'],
     authType: 'token',
     color: '#5865F2',
-    urlPattern: /^https?:\/\/(www\.)?discord\.com/,
+    // ptb/canary are Discord's beta clients — same channels, same token.
+    urlPattern: /^https?:\/\/((www|ptb|canary)\.)?discord\.com/,
     note: 'Open Discord in browser to capture token',
   },
 };
@@ -80,8 +81,23 @@ const PLATFORM_ARTIST_PATTERNS = {
   patreon:       /^https?:\/\/(www\.)?patreon\.com\/(?:cw\/|c\/)?(?!(?:home|search|messages|notifications|library|settings|posts)(?:[\/?#]|$))[^/?#]+/i,
   subscribestar: /^https?:\/\/(www\.)?subscribestar\.(com|adult)\/(?!feed$|messages$|library$)[^/?#]+\/?$/i,
   hentaifoundry: /^https?:\/\/(www\.)?hentai-foundry\.com\/user\/[^/?#]+/i,
+  // A Discord URL names a server or channel, not a creator: the backend's slug
+  // is `<server>[/<channel>]` and the Add panel asks which artist it belongs
+  // to. A message jump link still names its channel; DMs (@me) and thread
+  // links don't match. Mirrors extension_service._PLATFORM_PATTERNS.
+  discord:       /^https?:\/\/(?:www\.|ptb\.|canary\.)?discord\.com\/channels\/\d+(?:\/\d+)?(?:\/\d+)?\/?(?:[?#].*)?$/i,
 };
 
+/**
+ * `{serverId, channelId}` from a Discord channel/server URL the artist
+ * pattern accepts, else null. channelId is null for a whole-server URL.
+ */
+function parseDiscordUrl(url) {
+  if (!PLATFORM_ARTIST_PATTERNS.discord.test(url || '')) return null;
+  const m = /\/channels\/(\d+)(?:\/(\d+))?/.exec(url);
+  return m ? { serverId: m[1], channelId: m[2] || null } : null;
+}
+
 function getPlatformFromUrl(url) {
   for (const [key, platform] of Object.entries(PLATFORMS)) {
     if (platform.urlPattern.test(url)) return key;