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/test/chip.spec.js

@@ -198,3 +198,46 @@ describe('Add panel on Patreon and SubscribeStar', () => {
     expect(c.artistName).toBe('Tamada')
   })
 })
+
+const { posterLabel, initialPosterPick, postersRequest, withSelected } = loadLib('chip.js', [
+  'posterLabel', 'initialPosterPick', 'postersRequest', 'withSelected',
+])
+
+describe('the poster checklist rows', () => {
+  it('adds someone on the list who has not posted lately, by their saved name', () => {
+    const rows = withSelected({
+      posters: [{ id: '7', username: 'todding', global_name: 'Todding' }],
+      selected: ['7', '42'],
+      selected_labels: { 42: 'Old Name' },
+    })
+    expect(rows.map((r) => r.id)).toEqual(['7', '42'])
+    expect(posterLabel(rows[1])).toBe('Old Name')
+    expect(rows[1].reasons).toEqual(['on the list, not posting lately'])
+  })
+})
+
+describe('the Discord poster picker (#4488)', () => {
+  const posters = [
+    { id: '7', username: 'todding', global_name: 'Todding', suggested: true },
+    { id: '8', username: 'jakeboii', global_name: 'Jake Boii', suggested: false },
+    { id: '9', username: 'plain', global_name: null, suggested: false },
+  ]
+
+  it('labels a poster by display name, then handle', () => {
+    expect(posterLabel(posters[0])).toBe('Todding (@todding)')
+    expect(posterLabel(posters[2])).toBe('plain')
+  })
+
+  it('starts from the source list when it has one, else the suggestion', () => {
+    expect([...initialPosterPick({ posters, selected: [] })]).toEqual(['7'])
+    expect([...initialPosterPick({ posters, selected: ['8'] })]).toEqual(['8'])
+    expect([...initialPosterPick({ posters: [] })]).toEqual([])
+  })
+
+  it('sends ids with the names they were picked under, keeping unknown ids', () => {
+    expect(postersRequest(posters, new Set(['7', '42']))).toEqual([
+      { id: '7', name: 'Todding' },
+      { id: '42', name: null },
+    ])
+  })
+})