feat(e2ee): tell the user when their device's key uploads are stuck (#201 KE-1)

When the server keeps rejecting this device's /keys/upload with 400 "One
time key ... already exists", the device's encryption store has diverged
and the SDK retries forever (matrix-rust-sdk#5200, still open). Synapse
saw 474k of these from May 30 to Jul 8 2026. The fix is a fresh login, so:

- keyUploadWatch observes /keys/upload responses through the client's
  fetchFn (the response the SDK gets is untouched; a clone is read).
  Three conflicts within 15 min = stuck; any successful upload clears it.
- KeyUploadBanner (critical strip, same slot as the sync/clock banners):
  "Encryption on this device is out of sync" with Details and
  "Fix: sign out…", which opens the normal logout dialog (it already
  warns when the device isn't verified / backed up).

Server side, Grafana now alerts on the same signal (E2EE Key Upload
Conflicts) and on delayed-event 5xx (Call Delayed-Event Errors).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/keyUploadWatch.ts

@@ -0,0 +1,102 @@
+/**
+ * [Gitea #201 KE-1] Notice when this device's encryption store has diverged
+ * from the server: every POST /keys/upload answers 400 "One time key ...
+ * already exists", and the SDK retries forever (matrix-rust-sdk#5200, open),
+ * so new encrypted messages and calls quietly degrade. The server saw 474k of
+ * these between May 30 and Jul 8 2026. The only fix is a fresh login on that
+ * device, so we tell the user (KeyUploadBanner) instead of looping silently.
+ *
+ * Watched at the fetch layer (MatrixClient `fetchFn`), so it doesn't depend on
+ * SDK log wording. Pure state + a tiny subscribe API; no network I/O of its own.
+ */
+
+/** Conflicts needed before we call the device stuck. The SDK retries within
+ * seconds, so a real divergence crosses this quickly; a one-off race doesn't. */
+export const STUCK_AFTER = 3;
+/** Conflicts older than this don't count (a long-ago blip isn't "now"). */
+export const WINDOW_MS = 15 * 60 * 1000;
+
+export type KeyUploadWatchState = { stuck: boolean; conflicts: number; lastError?: string };
+
+let conflictTimes: number[] = [];
+let lastError: string | undefined;
+let state: KeyUploadWatchState = { stuck: false, conflicts: 0 };
+const listeners = new Set<() => void>();
+
+const publish = (next: KeyUploadWatchState) => {
+  if (next.stuck === state.stuck && next.conflicts === state.conflicts) return;
+  state = next;
+  listeners.forEach((l) => l());
+};
+
+export const isKeyUploadUrl = (url: string): boolean => {
+  try {
+    return /\/_matrix\/client\/[^/]+\/keys\/upload\/?$/.test(new URL(url, 'http://x').pathname);
+  } catch {
+    return false;
+  }
+};
+
+/** The 400 body that marks a diverged one-time-key store. */
+export const isOtkConflict = (status: number, body: string): boolean =>
+  status === 400 && /one time key .*already exists/i.test(body);
+
+/** Feed one /keys/upload response. `now` is injectable for tests. */
+export const recordKeyUpload = (status: number, body: string, now = Date.now()): void => {
+  if (status >= 200 && status < 300) {
+    // A successful upload means the stores agree again.
+    conflictTimes = [];
+    lastError = undefined;
+    publish({ stuck: false, conflicts: 0 });
+    return;
+  }
+  if (!isOtkConflict(status, body)) return;
+  conflictTimes = [...conflictTimes, now].filter((t) => now - t <= WINDOW_MS);
+  lastError = body.slice(0, 300);
+  publish({
+    stuck: conflictTimes.length >= STUCK_AFTER,
+    conflicts: conflictTimes.length,
+    lastError,
+  });
+};
+
+export const getKeyUploadWatch = (): KeyUploadWatchState => state;
+
+export const subscribeKeyUploadWatch = (cb: () => void): (() => void) => {
+  listeners.add(cb);
+  return () => listeners.delete(cb);
+};
+
+/** Test/logout helper: forget everything (a new session starts clean). */
+export const resetKeyUploadWatch = (): void => {
+  conflictTimes = [];
+  lastError = undefined;
+  publish({ stuck: false, conflicts: 0 });
+};
+
+const urlOf = (input: RequestInfo | URL): string =>
+  typeof input === 'string' ? input : input instanceof URL ? input.href : input.url;
+
+/**
+ * Wrap a fetch so /keys/upload responses are observed. The response the SDK
+ * gets is untouched; a clone is read in the background, never awaited.
+ */
+export const watchKeyUploads =
+  (inner: typeof fetch): typeof fetch =>
+  async (input, init) => {
+    const res = await inner(input, init);
+    try {
+      if (isKeyUploadUrl(urlOf(input))) {
+        if (res.ok) recordKeyUpload(res.status, '');
+        else if (res.status === 400)
+          res
+            .clone()
+            .text()
+            .then((body) => recordKeyUpload(res.status, body))
+            .catch(() => undefined);
+      }
+    } catch {
+      // observation must never break the request
+    }
+    return res;
+  };