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/composables/useApi.js

@@ -0,0 +1,50 @@
+// Thin fetch wrapper. Centralized so every call has consistent error handling
+// and json parsing. Errors throw an ApiError with the response status + body.
+
+export class ApiError extends Error {
+  constructor(message, status, body) {
+    super(message)
+    this.name = 'ApiError'
+    this.status = status
+    this.body = body
+  }
+}
+
+async function request(method, url, { body, params, signal } = {}) {
+  let fullUrl = url
+  if (params) {
+    const search = new URLSearchParams()
+    for (const [k, v] of Object.entries(params)) {
+      if (v !== undefined && v !== null) search.set(k, String(v))
+    }
+    const qs = search.toString()
+    if (qs) fullUrl += (url.includes('?') ? '&' : '?') + qs
+  }
+
+  const headers = {}
+  const init = { method, headers, signal }
+  if (body !== undefined) {
+    headers['Content-Type'] = 'application/json'
+    init.body = JSON.stringify(body)
+  }
+
+  const r = await fetch(fullUrl, init)
+  const text = await r.text()
+  let parsed
+  try { parsed = text ? JSON.parse(text) : null } catch { parsed = text }
+
+  if (!r.ok) {
+    const msg = (parsed && parsed.error) || `${r.status} ${r.statusText}`
+    throw new ApiError(msg, r.status, parsed)
+  }
+  return parsed
+}
+
+export function useApi() {
+  return {
+    get:    (url, opts) => request('GET',    url, opts),
+    post:   (url, opts) => request('POST',   url, opts),
+    patch:  (url, opts) => request('PATCH',  url, opts),
+    delete: (url, opts) => request('DELETE', url, opts)
+  }
+}