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/js/app.js

@@ -18,7 +18,8 @@ import { registerShortcut, guards, renderShortcutHelp } from './modules/shortcut
 import { openGallery, closeGallery } from './modules/gallery.js';
 import { refreshTasksView, openTasks, closeTasks } from './modules/tasks.js';
 import { openCalendar, closeCalendar, refreshCalendar } from './modules/calendar.js';
-import { startBackgroundSync, triggerSync, loadSettings as loadCaldavSettings, loadRelaySettings } from './modules/caldav.js';
+import { loadSettings as loadCaldavSettings } from './modules/caldav.js';
+import { startBackgroundSync, triggerSync, loadRelaySettings } from './modules/sync.js';
 import { openPluginSettings } from './modules/plugin-settings.js';
 
 // ── Global API (for inline onclick handlers and cross-module calls) ──