feat(fc2a): add TimelineSidebar and assemble GalleryView

GalleryView wires the grid + timeline + modal together. The URL is the
source of truth for which image (if any) is open — clicking an item pushes
?image=N; closing the modal pops it. This makes deep links shareable and
the back button work intuitively.

Below 900px viewport, the timeline drops below the grid as a horizontal
strip so it doesn't compete with thumbnail space on mobile.

Modal store + ImageViewer are placeholders here; real implementations land
in Batch 5 (Tasks 19–22).

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

frontend/src/router.js

@@ -1,10 +1,11 @@
 import { createRouter, createWebHistory } from 'vue-router'
 import PlaceholderView from './views/PlaceholderView.vue'
 import SettingsView from './views/SettingsView.vue'
+import GalleryView from './views/GalleryView.vue'
 
 const routes = [
   // FC-2: image backbone
-  { path: '/',          name: 'gallery',       component: PlaceholderView, meta: { title: 'Gallery'      } },
+  { path: '/',          name: 'gallery',       component: GalleryView,    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: SettingsView,    meta: { title: 'Settings'     } },