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.ts

@@ -0,0 +1,73 @@
+import { MatrixClient, MatrixError } from 'matrix-js-sdk';
+
+/**
+ * Synapse rate-limits `PUT /presence/{user}/status` to ONE request per 10 s per
+ * user by default (`rc_presence.per_user`: 0.1/s, burst 1) — and the limit is
+ * shared across all of a user's devices. Our presence heartbeat (online/away on
+ * visibility and activity changes) can therefore make a manual status save 429.
+ *
+ * `setPresenceWithRetry` waits out `Retry-After` (bounded) instead of failing,
+ * and `PresenceDeduper` lets the heartbeat skip writes that would not change
+ * anything, so it stops spending the budget in the first place.
+ */
+export type PresenceWrite = { presence: 'online' | 'offline' | 'unavailable'; status_msg?: string };
+
+const sleep = (ms: number) =>
+  new Promise<void>((r) => {
+    setTimeout(r, ms);
+  });
+
+export async function setPresenceWithRetry(
+  mx: Pick<MatrixClient, 'setPresence'>,
+  write: PresenceWrite,
+  opts: { maxWaitMs?: number; sleepFn?: (ms: number) => Promise<void> } = {},
+): Promise<void> {
+  const maxWaitMs = opts.maxWaitMs ?? 25_000;
+  const sleepFn = opts.sleepFn ?? sleep;
+  let waited = 0;
+  for (;;) {
+    try {
+      // eslint-disable-next-line no-await-in-loop
+      await mx.setPresence(write);
+      return;
+    } catch (e) {
+      if (!(e instanceof MatrixError) || e.httpStatus !== 429) throw e;
+      const wait = Math.min(e.getRetryAfterMs() ?? 10_500, 15_000);
+      if (waited + wait > maxWaitMs) throw e;
+      waited += wait;
+      // eslint-disable-next-line no-await-in-loop
+      await sleepFn(wait);
+    }
+  }
+}
+
+const presenceKey = (write: PresenceWrite): string => `${write.presence}|${write.status_msg ?? ''}`;
+
+/** Remembers the last write that succeeded so identical heartbeats are skipped. */
+export class PresenceDeduper {
+  private last: string | null = null;
+
+  shouldSend(write: PresenceWrite): boolean {
+    return presenceKey(write) !== this.last;
+  }
+
+  sent(write: PresenceWrite): void {
+    this.last = presenceKey(write);
+  }
+
+  reset(): void {
+    this.last = null;
+  }
+}
+
+/** A user-facing sentence for a failed status save. */
+export const describePresenceError = (e: unknown): string => {
+  if (e instanceof MatrixError) {
+    if (e.httpStatus === 429) {
+      return 'The server limits how often presence can change (once every 10 seconds, shared by all your devices). Please wait a moment and try again.';
+    }
+    const serverText = (e.data as { error?: string } | undefined)?.error;
+    if (serverText) return `Failed to save status: ${serverText}`;
+  }
+  return "Failed to save status — couldn't reach the server. Try again.";
+};