fix(notifications): timed mutes are lifted by any Lotus device, not just the one that set them (#333)

A timed mute writes a server-side push rule (muted everywhere), but the
unmute timer lived in one browser's localStorage: close that laptop and
the room stayed muted on the phone until it was reopened, and forever
after a logout there. Timers now live in account data
(io.lotus.mute_timers: { timers: { [roomId]: unmuteAt } }). Every
client schedules from it, reschedules when it changes, and the first one
online after expiry unmutes. An indefinite mute clears a pending timer.
Local timers from before are migrated once.

Fixes #333

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

src/app/state/plaintextCaches.ts

@@ -64,9 +64,10 @@ const clearMsgDrafts = (): void => {
  *   `lotus-status-expiry-*`, used to be preserved; since [Gitea #204] they are
  *   swept with the rest — the server-side presence status survives, so a
  *   re-login loses nothing)
- * - low-sensitivity UI/metadata residue (`io.lotus.mute_timers`, collapsed
- *   nav/space categories, `cinny_oidc_dynamic_clients`) is treated as
- *   preferences, not swept here
+ * - low-sensitivity UI/metadata residue (collapsed nav/space categories,
+ *   `cinny_oidc_dynamic_clients`) is treated as preferences, not swept here.
+ *   Timed mutes moved to account data in [Gitea #333]; a leftover local
+ *   `io.lotus.mute_timers` is migrated there on the next start.
  */
 /**
  * [Gitea #204] The local mirror of the user's status message (+ its expiry) is