fix(threads): viewing a room no longer reads threads you don't follow (#314)

A passive read (just looking at the room) sent a threaded receipt for
every unread thread you don't follow, so the Threads panel's Unread
filter and Mark all read only ever worked for followed threads.
Decision (2026-10-19): leave them unread until opened, but stop
counting unfollowed threads toward the room's unread dot
(getUnreadInfo subtracts them like muted threads; a mention makes a
thread followed, so highlights are never hidden). isThreadFollowed
moves to utils/room. Adds a Playwright regression.

Fixes #314

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/room.test.ts

@@ -928,3 +928,42 @@ test('getAllVersionsRoomCreator returns empty set when no create event', () => {
 test('EventTimeline.FORWARDS constant is available', () => {
   assert.equal(typeof EventTimeline.FORWARDS, 'string');
 });
+
+test('getUnreadInfo: unfollowed threads do not light the room; followed and mentioned do (#314)', () => {
+  const threads = [
+    { id: '$followed', hasCurrentUserParticipated: true },
+    { id: '$other' },
+    { id: '$mention' },
+  ];
+  const counts: Record<string, { total: number; highlight: number }> = {
+    $followed: { total: 2, highlight: 0 },
+    $other: { total: 3, highlight: 0 },
+    $mention: { total: 1, highlight: 1 },
+  };
+  const room = {
+    roomId: '!r:x',
+    // Server room totals include thread counts: 2 + 3 + 1 thread, 0 main.
+    getUnreadNotificationCount: (type: NotificationCountType) =>
+      type === NotificationCountType.Total ? 6 : 1,
+    getThreads: () => threads,
+    getThreadUnreadNotificationCount: (id: string, type: NotificationCountType) =>
+      type === NotificationCountType.Total ? counts[id].total : counts[id].highlight,
+    getEventReadUpTo: () => null,
+    getLiveTimeline: () => ({ getEvents: () => [] }),
+  } as unknown as Room;
+  const me = { getUserId: () => '@me:x' } as unknown as MatrixClient;
+  assert.deepEqual(getUnreadInfo(room, undefined, me), { roomId: '!r:x', highlight: 1, total: 3 });
+  // Only an unfollowed thread is unread: the room isn't lit.
+  counts.$followed.total = 0;
+  counts.$mention = { total: 0, highlight: 0 };
+  const onlyOther = {
+    ...room,
+    getUnreadNotificationCount: (type: NotificationCountType) =>
+      type === NotificationCountType.Total ? 3 : 0,
+  } as unknown as Room;
+  assert.deepEqual(getUnreadInfo(onlyOther, undefined, me), {
+    roomId: '!r:x',
+    highlight: 0,
+    total: 0,
+  });
+});