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/index.php

@@ -1834,9 +1834,9 @@ if (str_starts_with($route, '/api/')) {
                             <div class="font-pixel text-[9px] uppercase tracking-wider text-t-dim mb-2"><i class="ph ph-timer text-xs"></i> sync frequency</div>
                             <p class="text-[10px] text-t-dim leading-relaxed mb-2">How often the background sync polls the relay + CalDAV for new messages, collab updates and sync events. Lower = snappier, higher = less network traffic.</p>
                             <div class="flex items-center gap-3">
-                                <label class="text-xs text-t-muted flex-1" for="setting-sync-interval">Poll every</label>
-                                <input id="setting-sync-interval" type="number" min="5" max="3600" step="1" class="bg-t-bg border border-t-border text-t-bright text-xs font-mono px-2 py-1 rounded-sm focus:border-t-accent focus:outline-none" style="width:72px;text-align:right;">
-                                <span class="text-xs text-t-dim">sec</span>
+                                <label class="text-xs text-t-muted" for="setting-sync-interval">Poll every</label>
+                                <input id="setting-sync-interval" type="range" min="0" max="14" step="1" class="design-slider">
+                                <span id="setting-sync-interval-val" class="text-[11px] text-t-muted" style="min-width:52px;text-align:right;flex-shrink:0;">15s</span>
                             </div>
                         </div>
                     </div>