fix(sidebar): exclude watch_folders heartbeat from active task count

The watcher worker reports its periodic watch_folders task as
perpetually active, which kept the sidebar background-activity
spinner running even when no real work was in flight.

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

frontend/src/hooks/useScanActivity.ts

@@ -20,11 +20,21 @@ function pendingTasks(ws: WorkerStatus | undefined): number {
 
 /** Tasks actively being executed by workers right now. Distinct from
  *  pendingTasks so we can drop the spinner the moment no real work is
- *  in flight, even if reserved tasks linger. */
+ *  in flight, even if reserved tasks linger.
+ *
+ *  Excludes the periodic `watch_folders` heartbeat — the watcher
+ *  worker reports it as a perpetually-active task, which would pin
+ *  the spinner on indefinitely with no real work in flight. */
+const IDLE_TASK_NAMES = new Set(['watch_folders'])
 function activeTasks(ws: WorkerStatus | undefined): number {
   if (!ws) return 0
   return (
-    ws.workers?.reduce((sum, w) => sum + (w.active ?? 0), 0) ?? 0
+    ws.workers?.reduce((sum, w) => {
+      const real = (w.active_tasks ?? []).filter(
+        (t) => !IDLE_TASK_NAMES.has(t.name),
+      ).length
+      return sum + real
+    }, 0) ?? 0
   )
 }