chore(fc2a): polish pass — toasts, loading skeletons, focus guards, mobile breakpoints

AppSnackbar mounts once at the app root and exposes a window.__fcToast
function the stores call from error paths. This avoids prop-drilling a
toast emitter through every component while keeping the snackbar
component itself testable in isolation.

GalleryGrid renders shimmer-skeleton placeholders on initial load so the
first paint isn't empty. Skeleton uses the same auto-fill grid columns
as the real items so layout doesn't shift when content arrives.

Modal arrow nav now ignores text-entry elements so typing in the tag
autocomplete doesn't navigate prev/next. Small-screen breakpoint
(<600px) tightens gallery thumbnails to 120px min.

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

frontend/src/App.vue

@@ -3,9 +3,19 @@
     <AppShell>
       <RouterView />
     </AppShell>
+    <AppSnackbar ref="snackbar" />
   </v-app>
 </template>
 
 <script setup>
+import { onMounted, ref } from 'vue'
 import AppShell from './components/AppShell.vue'
+import AppSnackbar from './components/AppSnackbar.vue'
+
+const snackbar = ref(null)
+
+onMounted(() => {
+  // Expose snackbar via a simple global so stores can call it without props.
+  window.__fcToast = (opts) => snackbar.value?.open(opts)
+})
 </script>