feat(I3): always-on pipeline status indicator in the top nav

New /api/system/activity/summary aggregates scheduler health + per-queue
pending depths + running count + 24h failure count in one cached call (safe
to poll app-wide). PipelineStatusChip lives in the TopNav on every page: a
compact running/queued/failing chip with a scheduler-health dot that expands
to a popover (scheduler, busy queues, counts, link to downloads). Polls the
summary every 8s, paused when backgrounded. Reuses the queue-cache read via
_queues_cached(). + API test for the summary shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/stores/systemActivity.js

@@ -93,11 +93,23 @@ export const useSystemActivityStore = defineStore('systemActivity', () => {
     runsFilter.value = { ...runsFilter.value, ...filter }
   }
 
+  // One-call rollup for the always-on TopNav pipeline indicator:
+  // { scheduler, queues, queued_total, running, failing }.
+  const summary = ref(null)
+  async function loadSummary() {
+    try {
+      summary.value = await api.get('/api/system/activity/summary')
+    } catch (e) {
+      lastError.value = e.message
+    }
+    return summary.value
+  }
+
   return {
-    queues, workers, recentMinute, failures,
+    queues, workers, recentMinute, failures, summary,
     runs, runsCursor, runsHasMore, runsFilter,
     loading, lastError,
     loadQueues, loadWorkers, loadRecentMinute,
-    loadRuns, loadFailures, setFilter,
+    loadRuns, loadFailures, loadSummary, setFilter,
   }
 })