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/useThread.ts

@@ -4,7 +4,6 @@ import {
   EventTimeline,
   MatrixClient,
   MatrixEvent,
-  ReceiptType,
   Room,
   RoomEvent,
   RoomEventHandlerMap,
@@ -146,32 +145,6 @@ export const useThreadPendingEvents = (
   return pending;
 };
 
-/**
- * Send a threaded read receipt up to the latest confirmed event in the thread.
- *
- * The receipt is threaded by default (scoped to this thread), which clears the
- * per-thread unread count. Mirrors the latest-valid-event scan in
- * `utils/notifications.ts`.
- */
-export const markThreadAsRead = async (
-  mx: MatrixClient,
-  thread: Thread,
-  privateReceipt: boolean,
-): Promise<void> => {
-  const events = thread.liveTimeline.getEvents();
-
-  let latestEvent: MatrixEvent | undefined;
-  for (let i = events.length - 1; i >= 0; i -= 1) {
-    const evt = events[i];
-    if (evt && !evt.isSending()) {
-      latestEvent = evt;
-      break;
-    }
-  }
-  if (!latestEvent) return;
-
-  await mx.sendReadReceipt(
-    latestEvent,
-    privateReceipt ? ReceiptType.ReadPrivate : ReceiptType.Read,
-  );
-};
+// markThreadAsRead moved to ./threadReceipt (pure + unit-tested); re-exported
+// here for existing import sites.
+export { markThreadAsRead } from './threadReceipt';