feat: the extension picks a Discord source's posters from who posts in the channel, stored by id (#4488)

On a Discord channel page the Add panel lists who posted in the newest 200
messages, the creator the server is named for (or owned by) ticked. On a
channel FabledCurator already follows, the chip opens the same list with Save
and Open artist. The list is kept as user ids, so a rename never quietly stops
it matching; the name each was picked under rides beside it for display, and
the Subscriptions dialog shows it.

- DiscordClient.recent_posters + rank_posters (owner / name-matches-server;
  bots never suggested; it suggests, the operator ticks).
- GET/POST /api/extension/discord/posters; quick-add takes discord_authors.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extension/lib/chip.js

@@ -144,3 +144,49 @@ function inlineCompletion(typed, results) {
   if (!t) return null;
   return (results || []).find((a) => a.name.length > t.length && a.name.toLowerCase().startsWith(t)) || null;
 }
+
+// ---- Discord poster picker (#4488) ----
+// /api/extension/discord/posters lists who posted in the channel lately, with
+// the creator the server is named for (or owned by) marked `suggested`. The
+// picker ticks posters; the source keeps their ids, since a username can
+// change on a whim and an id can't.
+
+/** "Todding (@todding)" — the display name, then the handle when it differs. */
+function posterLabel(p) {
+  const shown = p?.global_name || p?.username || p?.id || '';
+  const handle = p?.username && p.username !== shown ? ` (@${p.username})` : '';
+  return `${shown}${handle}`;
+}
+
+/** What the picker starts with: the source's current list when it has one,
+ *  else the suggested creator(s). A set of ids. */
+function initialPosterPick(data) {
+  const current = (data?.selected || []).map(String);
+  if (current.length) return new Set(current);
+  return new Set((data?.posters || []).filter((p) => p.suggested).map((p) => String(p.id)));
+}
+
+/** The checklist's rows: who posted lately, then anyone already on the
+ *  source's list who hasn't — by the name they were picked under — so they
+ *  can still be unticked. */
+function withSelected(data) {
+  const posters = [...(data?.posters || [])];
+  const shown = new Set(posters.map((p) => String(p.id)));
+  const labels = data?.selected_labels || {};
+  for (const id of (data?.selected || []).map(String)) {
+    if (shown.has(id)) continue;
+    posters.push({ id, global_name: labels[id] || null, username: null,
+      messages: 0, images: 0, reasons: ['on the list, not posting lately'] });
+  }
+  return posters;
+}
+
+/** The picked posters as the API takes them: [{id, name}], in list order.
+ *  An id already on the source but no longer posting lately is kept. */
+function postersRequest(posters, picked) {
+  const byId = new Map((posters || []).map((p) => [String(p.id), p]));
+  return [...picked].map((id) => ({
+    id,
+    name: byId.has(id) ? (byId.get(id).global_name || byId.get(id).username || id) : null,
+  }));
+}