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/modules/calendar.js

@@ -3,7 +3,8 @@
 import { $, esc } from './util.js';
 import { state } from './state.js';
 import { stripMd, getTodos } from './markdown.js';
-import { autoSync, getCalConfig, getAllCalConfigs } from './caldav.js';
+import { autoSync } from './sync.js';
+import { getCalConfig, getAllCalConfigs } from './caldav.js';
 import { renderCardHtml } from './board.js';
 
 const overlay = $('calendar');