fix(status): status save no longer fails on Synapse's 1-per-10s presence limit

Reported: 'Failed to save status — server may be rate limiting. Try again.'
on the first status change in a month. Root cause: Synapse rate-limits
PUT /presence/{user}/status to ONE request per 10 s per user by default
(rc_presence.per_user: per_second 0.1, burst_count 1), shared across all of
the user's devices, and our presence heartbeat (online/away on visibility and
activity changes, from every open tab/device) spends that budget — so a manual
save that lands within 10 s of a heartbeat gets a 429, which the form showed
as a dead end.

Two fixes: (1) the status save waits out Retry-After (bounded to ~25 s) via
setPresenceWithRetry instead of failing, and the error text now says what
actually happened (rate limit / server text / offline); (2) the heartbeat
dedupes — it only sends when presence or status actually changes (/sync
already keeps us online), so it stops burning the budget in the first place.
Unit-tested; reproduced headless with a routed 10 s limiter: heartbeat ok →
save 429 → retried 8 s later → saved.

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

src/app/utils/presenceWrite.test.ts

@@ -0,0 +1,81 @@
+import { test } from 'node:test';
+import assert from 'node:assert/strict';
+import { MatrixError } from 'matrix-js-sdk';
+import { PresenceDeduper, describePresenceError, setPresenceWithRetry } from './presenceWrite';
+
+const limited = (retryMs = 3000) =>
+  new MatrixError(
+    { errcode: 'M_LIMIT_EXCEEDED', error: 'Too Many Requests', retry_after_ms: retryMs },
+    429,
+  );
+
+test('429 is retried after Retry-After, then succeeds', async () => {
+  let n = 0;
+  const waits: number[] = [];
+  const mx = {
+    setPresence: async () => {
+      n += 1;
+      if (n < 3) throw limited(3000);
+      return {};
+    },
+  };
+  await setPresenceWithRetry(
+    mx,
+    { presence: 'online', status_msg: 'hi' },
+    {
+      sleepFn: async (ms) => {
+        waits.push(ms);
+      },
+    },
+  );
+  assert.equal(n, 3);
+  assert.deepEqual(waits, [3000, 3000]);
+});
+
+test('gives up once the total wait would exceed the cap; other errors are not retried', async () => {
+  const mx = {
+    setPresence: async () => {
+      throw limited(20_000);
+    },
+  };
+  await assert.rejects(
+    () =>
+      setPresenceWithRetry(
+        mx,
+        { presence: 'online' },
+        { maxWaitMs: 25_000, sleepFn: async () => undefined },
+      ),
+    /Too Many/,
+  );
+  let calls = 0;
+  const forbidden = {
+    setPresence: async () => {
+      calls += 1;
+      throw new MatrixError({ errcode: 'M_FORBIDDEN', error: 'nope' }, 403);
+    },
+  };
+  await assert.rejects(() => setPresenceWithRetry(forbidden, { presence: 'online' }));
+  assert.equal(calls, 1);
+});
+
+test('deduper skips identical writes and notices changes', () => {
+  const d = new PresenceDeduper();
+  assert.equal(d.shouldSend({ presence: 'online', status_msg: 'a' }), true);
+  d.sent({ presence: 'online', status_msg: 'a' });
+  assert.equal(d.shouldSend({ presence: 'online', status_msg: 'a' }), false);
+  assert.equal(d.shouldSend({ presence: 'unavailable', status_msg: 'a' }), true);
+  assert.equal(d.shouldSend({ presence: 'online', status_msg: 'b' }), true);
+  d.reset();
+  assert.equal(d.shouldSend({ presence: 'online', status_msg: 'a' }), true);
+});
+
+test('error wording', () => {
+  assert.match(describePresenceError(limited()), /once every 10 seconds/);
+  assert.equal(
+    describePresenceError(
+      new MatrixError({ errcode: 'M_FORBIDDEN', error: 'Presence is disabled' }, 403),
+    ),
+    'Failed to save status: Presence is disabled',
+  );
+  assert.match(describePresenceError(new TypeError('Failed to fetch')), /couldn't reach/);
+});