fix(correctness): call-join reset on embed swap + per-path notify dedupe

COR-2 (useCallEmbed): useCallJoined only reset `joined` when the embed became
undefined. Answering a 2nd call swaps the embed A->B directly (embed stays
truthy), so `joined` stayed true and call B rendered as already-joined,
skipping the loading/watchdog UI. Re-seed from `embed?.joined ?? false` on
every embed identity change.

COR-4 (ClientNonUIFeatures): the notify-dedupe used one Map<roomId,eventId>
slot shared by the main-timeline and per-thread paths, so a thread reply
overwrote the room's slot and a re-fired main message (decrypt/edit re-emit,
common in E2EE) then mismatched and double-notified. Key the slot by
`${roomId}|${threadId ?? 'main'}` so each path dedupes independently.

Both verified correct by two review passes (no missed-notification or
missed-join regressions).

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

src/app/hooks/useCallEmbed.ts

@@ -149,9 +149,11 @@ export const useCallJoined = (embed?: CallEmbed): boolean => {
   );
 
   useEffect(() => {
-    if (!embed) {
-      setJoined(false);
-    }
+    // Re-seed from the (new) embed whenever it changes identity, not only when
+    // it becomes undefined. Answering a 2nd call swaps embed A→B directly (embed
+    // stays truthy), so a plain `if (!embed)` reset left `joined` stuck true and
+    // B rendered as already-joined, skipping the loading/watchdog UI.
+    setJoined(embed?.joined ?? false);
   }, [embed]);
 
   return joined;