extract sync orchestration into sync.js, unify on sync.tick heartbeat

caldav.js was carrying the core sync timer, autosync toggle, relay
settings and every poller hook alongside its actual CalDAV/calendar
config concerns. Split out:

- New sync.js owns the single background timer, header Sync button,
  autosync pause toggle, configurable interval, and emits `sync.tick`
  so plugins can fan out without their own setIntervals.
- collab-buckets + chat now subscribe to sync.tick instead of running
  their own pollers, so the manual Sync button refreshes them too and
  the Autosync pause actually pauses everything.
- Replace the number input for sync interval with a stepped slider
  (5s..1h snap points) and a live label, bumped to 1.9.2.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/plugins/tests/runner.js

@@ -5,7 +5,7 @@
 // Backup is created before and auto-restored after every run.
 
 import { getCsrf } from '../../js/modules/util.js';
-import { stopBackgroundSync, startBackgroundSync } from '../../js/modules/caldav.js';
+import { stopBackgroundSync, startBackgroundSync } from '../../js/modules/sync.js';
 
 // Suites live on window so two runner.js instances (one cached and loaded
 // by test files via bare imports, one fresh and loaded by loader.js with