fix(sidebar): favourite / low-priority changes move the room immediately

Home's categorisation memo keyed only on the room list, and nothing
observed m.tag changes. The SDK emits RoomEvent.Tags on the room and
re-emits it on the client (room.js addTags, sync.js reEmit); a small
client-level hook bumps a version that the memo depends on.

Fixes #20

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

src/app/hooks/useRoomTagsVersion.ts

@@ -0,0 +1,31 @@
+import { useEffect } from 'react';
+import { MatrixClient, RoomEvent } from 'matrix-js-sdk';
+import { useForceUpdate } from './useForceUpdate';
+
+/**
+ * Bumps a counter whenever any room's `m.tag` account data changes
+ * (favourite/low-priority add or remove via `mx.setRoomTag`/`deleteRoomTag`).
+ *
+ * `RoomEvent.Tags` is emitted on the individual `Room` by `Room.addTags()`
+ * (matrix-js-sdk `lib/models/room.js`) and re-emitted onto the `MatrixClient`
+ * itself by the sync loop (`lib/sync.js`, the `client.reEmitter.reEmit(room, [...
+ * RoomEvent.Tags ...])` call), so a single client-level listener here sees the
+ * change for every room without needing to attach/detach a per-room listener.
+ *
+ * Consumers should add the returned counter to the deps of any memo that
+ * derives favourite/low-priority categorisation from `room.tags`, so toggling
+ * a tag moves the room immediately instead of waiting for an unrelated
+ * re-render to pick up the mutated (but stale-looking) `Room` object.
+ */
+export const useRoomTagsVersion = (mx: MatrixClient): number => {
+  const [version, bumpVersion] = useForceUpdate();
+
+  useEffect(() => {
+    mx.on(RoomEvent.Tags, bumpVersion);
+    return () => {
+      mx.removeListener(RoomEvent.Tags, bumpVersion);
+    };
+  }, [mx, bumpVersion]);
+
+  return version;
+};