feat(fc2a): add SettingsView shell with Overview tab (SystemStatsCards)

Overview tab shows total images/tags/storage/pending/failed using
Fraunces for the big numbers. Inline alert nudges the operator to the
Import tab when pending tasks exist. Polls /api/system/stats every 5s
while the tab is visible (pauses when the document is hidden).

Import tab references three placeholders (TriggerPanel/FiltersForm/
TaskList); real implementations land in Task 15.

Router replaces the placeholder view for /settings with SettingsView.

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

frontend/src/router.js

@@ -1,12 +1,13 @@
 import { createRouter, createWebHistory } from 'vue-router'
 import PlaceholderView from './views/PlaceholderView.vue'
+import SettingsView from './views/SettingsView.vue'
 
 const routes = [
   // FC-2: image backbone
   { path: '/',          name: 'gallery',       component: PlaceholderView, meta: { title: 'Gallery'      } },
   { path: '/showcase',  name: 'showcase',      component: PlaceholderView, meta: { title: 'Showcase'     } },
   { path: '/tags',      name: 'tags',          component: PlaceholderView, meta: { title: 'Tags'         } },
-  { path: '/settings',  name: 'settings',      component: PlaceholderView, meta: { title: 'Settings'     } },
+  { path: '/settings',  name: 'settings',      component: SettingsView,    meta: { title: 'Settings'     } },
 
   // FC-3: subscription backbone
   { path: '/subscriptions', name: 'subscriptions', component: PlaceholderView, meta: { title: 'Subscriptions' } },