feat(fc2a): add useApi composable, import store, extend system store with stats

useApi centralizes fetch error handling — every API call throws ApiError
on non-2xx with the parsed body attached, so components don't repeat the
'check response.ok' boilerplate.

import store wraps import settings, batch status, task list (with cursor
pagination), and the trigger/retry/clear admin actions. system store
gains a refreshStats() that powers the Settings overview tab.

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

frontend/src/stores/system.js

@@ -1,18 +1,30 @@
 import { defineStore } from 'pinia'
 import { ref } from 'vue'
+import { useApi } from '../composables/useApi.js'
 
 export const useSystemStore = defineStore('system', () => {
+  const api = useApi()
   const healthy = ref(null)  // null=unknown, true=ok, false=down
+  const stats = ref(null)
+  const statsLoading = ref(false)
 
   async function refreshHealth() {
     try {
-      const r = await fetch('/api/health')
-      const body = await r.json()
-      healthy.value = r.ok && body.status === 'ok'
+      const body = await api.get('/api/health')
+      healthy.value = body.status === 'ok'
     } catch {
       healthy.value = false
     }
   }
 
-  return { healthy, refreshHealth }
+  async function refreshStats() {
+    statsLoading.value = true
+    try {
+      stats.value = await api.get('/api/system/stats')
+    } finally {
+      statsLoading.value = false
+    }
+  }
+
+  return { healthy, stats, statsLoading, refreshHealth, refreshStats }
 })