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/components/gallery/GalleryGrid.vue

@@ -1,5 +1,8 @@
 <template>
   <div class="fc-gallery-grid">
+    <div v-if="store.loading && store.images.length === 0" class="fc-gallery-grid__skeleton">
+      <div v-for="i in 12" :key="i" class="fc-gallery-grid__skeleton-item" />
+    </div>
     <template v-for="group in store.dateGroups" :key="`${group.year}-${group.month}`">
       <h2 class="fc-gallery-grid__date-header" :id="dateHeaderId(group)">
         {{ monthLabel(group.year, group.month) }}
@@ -96,4 +99,33 @@ function imagesForGroup(group) {
   padding: 32px 0;
   color: rgb(var(--v-theme-on-surface-variant, var(--v-theme-on-surface)));
 }
+.fc-gallery-grid__skeleton {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(160px, 1fr));
+  gap: 8px;
+}
+.fc-gallery-grid__skeleton-item {
+  aspect-ratio: 1;
+  background: linear-gradient(
+    90deg,
+    rgb(var(--v-theme-surface)) 0%,
+    rgb(var(--v-theme-surface-light)) 50%,
+    rgb(var(--v-theme-surface)) 100%
+  );
+  background-size: 200% 100%;
+  animation: fc-shimmer 1.4s infinite;
+  border-radius: 4px;
+}
+@keyframes fc-shimmer {
+  0%   { background-position: 200% 0; }
+  100% { background-position: -200% 0; }
+}
+
+@media (max-width: 600px) {
+  .fc-gallery-grid__items,
+  .fc-gallery-grid__skeleton {
+    grid-template-columns: repeat(auto-fill, minmax(120px, 1fr));
+    gap: 4px;
+  }
+}
 </style>