feat: add SPA app shell, router, system store, and route placeholders

Nav lists every FC-2 and FC-3 surface with a placeholder view that names
the sub-project that will fill it. System store polls /api/health on mount
and renders a status pip in the nav footer.

Favicon uses Curator's aged amber accent (#A87338).

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

frontend/src/router.js

@@ -0,0 +1,20 @@
+import { createRouter, createWebHistory } from 'vue-router'
+import PlaceholderView from './views/PlaceholderView.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'     } },
+
+  // FC-3: subscription backbone
+  { path: '/subscriptions', name: 'subscriptions', component: PlaceholderView, meta: { title: 'Subscriptions' } },
+  { path: '/credentials',   name: 'credentials',   component: PlaceholderView, meta: { title: 'Credentials'   } },
+  { path: '/downloads',     name: 'downloads',     component: PlaceholderView, meta: { title: 'Downloads'     } }
+]
+
+export default createRouter({
+  history: createWebHistory(),
+  routes
+})