feat(fc2a): add gallery Pinia store, GalleryItem, and EmptyState

Gallery store handles cursor pagination, date-group merging across pages,
tag filter, timeline buckets, and jump-to-month. Stale response detection
via an inflightId counter prevents out-of-order page additions when the
user toggles filters quickly.

GalleryItem is a square thumbnail card with hover/focus accent outline,
keyboard activation (Enter/Space → open), video badge for video MIME types,
and a broken-image fallback if the thumbnail URL fails to load.

EmptyState points operators to /settings?tab=import so the empty path is
self-guiding.

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

frontend/src/components/gallery/EmptyState.vue

@@ -0,0 +1,27 @@
+<template>
+  <div class="fc-empty">
+    <v-icon size="56" icon="mdi-image-off-outline" class="fc-empty__icon" />
+    <h2 class="fc-empty__title">No images yet</h2>
+    <p class="fc-empty__body">
+      Drop files into <code>/import</code> and run a scan from
+      <RouterLink to="/settings?tab=import">Settings → Import</RouterLink>.
+    </p>
+  </div>
+</template>
+
+<style scoped>
+.fc-empty {
+  text-align: center;
+  padding: 96px 24px;
+  color: rgb(var(--v-theme-on-surface));
+}
+.fc-empty__icon { color: rgb(var(--v-theme-accent)); margin-bottom: 16px; }
+.fc-empty__title {
+  font-family: 'Fraunces', Georgia, serif;
+  font-size: 24px;
+  font-weight: 500;
+  margin-bottom: 8px;
+}
+.fc-empty__body { color: rgb(var(--v-theme-on-surface-variant, var(--v-theme-on-surface))); }
+code { background: rgb(var(--v-theme-surface-light)); padding: 1px 6px; border-radius: 4px; }
+</style>