fix(notifications/threads): Wave-1 audit fixes (🔴 + web 🟠)

- T1 (🔴): markThreadAsRead no longer receipts the thread ROOT (a 2nd instance
  of the read-marker-corruption regression — opening a thread whose root is old
  re-lit the whole room). Extracted to a pure threadReceipt.ts + 5 regression
  tests.
- N1 (🔴): favicon/tab-title unread count now sums only leaf rooms (was double-
  counting every ancestor-space aggregate in roomToUnread).
- N2 (🔴): notifications/sounds dedupe on the event id, not the unread count —
  fixes "read a DM, next message never notifies again".
- T4 (🟠): the thread notification path no longer re-gates on the room count, so
  an explicit per-thread "All replies" override in a Mentions-only room fires.
- N3 (🟠): getUnreadInfos skips phantom {0,0} entries (muted-thread-only rooms no
  longer light the nav row / pollute unread filters).
- N4 (🟠): the Receipt handler recomputes unread instead of blanket-DELETE, so a
  threaded receipt can't wipe a room's valid main-timeline badge.
- T2 (🟠): thread "Jump to Latest" re-anchors the virtual window (was landing on
  a stale mid/old event).

Gates: tsc/eslint/prettier clean, build OK, 678 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/thread/ThreadTimeline.tsx

@@ -460,12 +460,17 @@ export function ThreadTimeline({ room, thread, editor }: ThreadTimelineProps) {
   }, [scrollToBottomCount]);
 
   const handleJumpToBottom = useCallback(() => {
+    // Re-anchor the virtual window at the thread tail first. While scrolled up,
+    // live replies deliberately don't extend the window, so without this the chip
+    // would scroll to the bottom of the STALE window (a mid/old event) instead of
+    // the newest reply. Mirrors the main timeline's handleJumpToLatest.
+    setTimeline(getInitialThreadTimeline(thread, getLinkedTimelines(thread.liveTimeline)));
     scrollToBottomRef.current.count += 1;
     scrollToBottomRef.current.smooth = true;
     // Flip atBottom so the layout effect re-runs (count re-read) and live
     // events resume sticking to the bottom.
     setAtBottom(true);
-  }, []);
+  }, [thread]);
 
   // Scroll in-place editor into view.
   useEffect(() => {