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

e2e/local-homeserver.spec.ts

@@ -265,6 +265,46 @@ test.describe('local homeserver regression', () => {
     await expect.poll(threadUnread).toBe(false);
   });
 
+  test("a thread I don't follow stays unread while I view the room (#314)", async ({ page }) => {
+    const room = await createRoom(alice, 'Unfollowed Thread Room', { invite: [bob.userId] });
+    await joinRoom(bob, room);
+    const inThread = (root: string) => ({
+      'm.relates_to': {
+        rel_type: 'm.thread',
+        event_id: root,
+        is_falling_back: true,
+        'm.in_reply_to': { event_id: root },
+      },
+    });
+    const root = await sendText(bob, room, "bob's thread root");
+    await loginUI(page, alice);
+    await openRoom(page, room);
+    await sendText(bob, room, 'reply in bob thread', inThread(root));
+    await sendText(bob, room, 'newer main message 314');
+    await expect(page.getByText('newer main message 314')).toBeVisible();
+    const threadUnread = async () => {
+      const filter = {
+        room: {
+          rooms: [room],
+          timeline: { limit: 1, unread_thread_notifications: true, not_types: [uniq('x.')] },
+        },
+      };
+      const sync = await api<{
+        rooms: {
+          join: Record<string, { unread_thread_notifications?: Record<string, unknown> }>;
+        };
+      }>(
+        'GET',
+        `/_matrix/client/v3/sync?timeout=0&filter=${enc(JSON.stringify(filter))}`,
+        alice.token,
+      );
+      return root in (sync.rooms.join[room]?.unread_thread_notifications ?? {});
+    };
+    await page.waitForTimeout(2000); // let any receipt the room view would send go out
+    // Viewing the room used to read every thread you don't follow (#314).
+    expect(await threadUnread()).toBe(true);
+  });
+
   test('timeline image opens the gallery lightbox (#219) @webkit', async ({ page }) => {
     const room = await createRoom(alice, 'Lightbox Room');
     const png = Buffer.from(