fix(scheduler): prevent auto-enable with wrong interval on page load

Root cause: GET /api/sync/schedule returned interval_minutes=null when
scheduler was stopped, causing dropdown to stay on first HTML option
(1 min). Setting .value programmatically could trigger onchange, sending
a second PUT with interval=1 right after the user's intended interval.

- GET schedule falls back to DB/default (10 min) when scheduler is off
- Add _schedulerLoading flag to block onchange during loadSchedulerStatus
- Default interval 10 min everywhere (was 5 in backend)
- Cache bust dashboard.js v=33

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

api/app/routers/sync.py

@@ -804,8 +804,12 @@ async def update_schedule(config: ScheduleConfig):
 
 @router.get("/api/sync/schedule")
 async def get_schedule():
-    """Get current scheduler status."""
-    return scheduler_service.get_scheduler_status()
+    """Get current scheduler status (falls back to DB for interval)."""
+    status = scheduler_service.get_scheduler_status()
+    if status["interval_minutes"] is None:
+        config = await sqlite_service.get_scheduler_config()
+        status["interval_minutes"] = int(config.get("interval_minutes", "10"))
+    return status
 
 
 @router.get("/api/settings")