Add task count to task list panel header

Show 'X todo / Y done' in the Tasks panel title bar.

Also includes prior uncommitted work:
- Pagination in fetch_tasks (maxResults=100 + pageToken loop)
- fetch_tasks_since for incremental pull sync
- SyncStats struct with version/last_sync/last_pull/changed counts
- Periodic push (30s) and pull (5min) sync engine
- event::poll(100ms) for non-blocking UI refresh
- Ctrl+R full sync (push + pull)
- refresh_if_needed() to reload data after background sync
- Retry mechanism (MAX_SYNC_RETRIES=3) for sync queue items
- HTTP status code checks in fetch_lists/fetch_tasks/fetch_tasks_since
- Fix move_task URL to use reqwest query()
- Remove CASCADE via replace_all_lists (use insert_list instead)
- has_pending_sync() to prevent pull during pending push

src/domain/models.rs

@@ -40,4 +40,7 @@ pub struct SyncQueueItem {
     pub list_id: String,
     pub payload: String,
     pub created_at: String,
+    pub retries: i32,
 }
+
+pub const MAX_SYNC_RETRIES: i32 = 3;