feat(threads): "Mark all read" in the threads list (#165)

A "Mark all read" chip appears in the Threads panel header whenever any
thread in the room has unread replies, and sends one threaded receipt per
unread thread (never the root — a root lives in the main timeline and a
receipt there would drag the MAIN read marker backwards, the P6
regression). Honours the private-receipt settings.

Verified: two threads with unread replies → the chip appears, the rows
say "unread" in their labels; after clicking, no unread rows and the chip
is gone.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/utils/markThreadsRead.ts

@@ -0,0 +1,29 @@
+import { MatrixClient, NotificationCountType, ReceiptType, Room } from 'matrix-js-sdk';
+
+/**
+ * [Gitea #165] Send a threaded read receipt for every thread in the room that
+ * has unread replies. Mirrors the per-thread half of `markAsRead`, including
+ * its rule: never fall back to the thread root (a root lives in the main
+ * timeline, so a receipt there would move the MAIN read marker backwards).
+ * Returns how many threads were marked.
+ */
+export async function markAllThreadsRead(
+  mx: MatrixClient,
+  room: Room,
+  privateReceipt: boolean,
+): Promise<number> {
+  const receiptType = privateReceipt ? ReceiptType.ReadPrivate : ReceiptType.Read;
+  const sends = room
+    .getThreads()
+    .map((thread) => {
+      const unread =
+        room.getThreadUnreadNotificationCount(thread.id, NotificationCountType.Total) ?? 0;
+      if (unread <= 0) return undefined;
+      const lastReply = thread.lastReply();
+      if (!lastReply || lastReply.isSending()) return undefined;
+      return mx.sendReadReceipt(lastReply, receiptType, false).catch(() => undefined);
+    })
+    .filter((p): p is NonNullable<typeof p> => !!p);
+  await Promise.all(sends);
+  return sends.length;
+}