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/components/modal/ImageViewer.vue

@@ -0,0 +1,9 @@
+<template>
+  <v-dialog :model-value="true" max-width="600" @update:model-value="$emit('close')">
+    <v-card>
+      <v-card-title>Image viewer</v-card-title>
+      <v-card-text>Modal viewer lands in Batch 5 (Tasks 19–22).</v-card-text>
+    </v-card>
+  </v-dialog>
+</template>
+<script setup>defineEmits(['close'])</script>