feat(fc3a): SubscriptionsView + ArtistSection + SourceRow + router

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

frontend/src/router.js

@@ -7,6 +7,7 @@ import TagsView from './views/TagsView.vue'
 import ArtistView from './views/ArtistView.vue'
 import SeriesManageView from './views/SeriesManageView.vue'
 import SeriesReaderView from './views/SeriesReaderView.vue'
+import SubscriptionsView from './views/SubscriptionsView.vue'
 
 // The application's front door. `/` redirects here. Changing the front door
 // is a one-line edit (e.g. '/gallery' or '/tags').
@@ -29,7 +30,7 @@ const routes = [
   { path: '/settings', name: 'settings', component: SettingsView, meta: { title: 'Settings' } },
 
   // FC-3: subscription backbone
-  { path: '/subscriptions', name: 'subscriptions', component: PlaceholderView, meta: { title: 'Subscriptions' } },
+  { path: '/subscriptions', name: 'subscriptions', component: SubscriptionsView, meta: { title: 'Subscriptions' } },
   { path: '/credentials',   name: 'credentials',   component: PlaceholderView, meta: { title: 'Credentials'   } },
   { path: '/downloads',     name: 'downloads',     component: PlaceholderView, meta: { title: 'Downloads'     } }
 ]