feat(threads): enable SDK threadSupport + unthreaded read receipts (P3-8 step 0)

threadSupport:true makes matrix-js-sdk partition m.thread relations into Thread
objects (replies leave the main timeline; roots stay). markAsRead now sends
UNTHREADED receipts so one receipt still clears room + thread notification
counts — without this, badges would stick unread. The thread panel + summary
chips land in the same push.

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

src/app/utils/notifications.ts

@@ -21,8 +21,13 @@ export async function markAsRead(mx: MatrixClient, roomId: string, privateReceip
   const latestEvent = getLatestValidEvent();
   if (latestEvent === null) return;
 
+  // Unthreaded receipt: with client threadSupport enabled the SDK would
+  // otherwise scope this to the main timeline (thread_id: "main"), leaving
+  // per-thread notification counts permanently unread. Unthreaded preserves
+  // the pre-threads wire behavior — one receipt clears everything.
   await mx.sendReadReceipt(
     latestEvent,
     privateReceipt || privateReadReceipts ? ReceiptType.ReadPrivate : ReceiptType.Read,
+    true,
   );
 }