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/views/GalleryView.vue

@@ -0,0 +1,77 @@
+<template>
+  <v-container fluid class="py-6">
+    <h1 class="fc-h1 mb-4">Gallery</h1>
+    <div class="fc-gallery-layout">
+      <div class="fc-gallery-layout__main">
+        <EmptyState v-if="store.isEmpty" />
+        <GalleryGrid v-else @open="openImage" />
+      </div>
+      <TimelineSidebar v-if="store.images.length > 0" class="fc-gallery-layout__sidebar" />
+    </div>
+
+    <ImageViewer
+      v-if="modal.currentImageId !== null"
+      @close="closeImage"
+    />
+  </v-container>
+</template>
+
+<script setup>
+import { onMounted, watch } from 'vue'
+import { useRoute, useRouter } from 'vue-router'
+import { useGalleryStore } from '../stores/gallery.js'
+import { useModalStore } from '../stores/modal.js'
+import GalleryGrid from '../components/gallery/GalleryGrid.vue'
+import TimelineSidebar from '../components/gallery/TimelineSidebar.vue'
+import EmptyState from '../components/gallery/EmptyState.vue'
+import ImageViewer from '../components/modal/ImageViewer.vue'
+
+const store = useGalleryStore()
+const modal = useModalStore()
+const router = useRouter()
+const route = useRoute()
+
+onMounted(async () => {
+  await store.loadInitial()
+  await store.loadTimeline()
+  // Open modal if URL has ?image=N
+  const initial = parseInt(route.query.image, 10)
+  if (!isNaN(initial)) modal.open(initial)
+})
+
+watch(() => route.query.image, (q) => {
+  const id = parseInt(q, 10)
+  if (!isNaN(id) && id !== modal.currentImageId) modal.open(id)
+  else if (isNaN(id) && modal.currentImageId !== null) modal.close()
+})
+
+function openImage(id) {
+  router.push({ query: { ...route.query, image: id } })
+}
+function closeImage() {
+  const q = { ...route.query }
+  delete q.image
+  router.push({ query: q })
+}
+</script>
+
+<style scoped>
+.fc-h1 {
+  font-family: 'Fraunces', Georgia, serif;
+  font-size: 32px;
+  font-weight: 500;
+  color: rgb(var(--v-theme-on-surface));
+}
+.fc-gallery-layout {
+  display: flex;
+  gap: 16px;
+  align-items: flex-start;
+}
+.fc-gallery-layout__main { flex: 1; min-width: 0; }
+.fc-gallery-layout__sidebar { flex-shrink: 0; }
+
+@media (max-width: 900px) {
+  .fc-gallery-layout { flex-direction: column-reverse; }
+  .fc-gallery-layout__sidebar { width: 100%; max-height: 200px; }
+}
+</style>