feat(gallery): visual 'more like this' UI (Phase 3 frontend)

Modal 'Related' strip (RelatedStrip.vue) — top-12 similar thumbs, fetched on
its own DEFERRED, single-flighted path (200ms after the modal is up) so it
never blocks or slows the modal; collapses silently on empty/slow/error and is
hidden when the image has no embedding (has_embedding flag). 'See all similar'
closes the modal and navigates the gallery to ?similar_to=<id>.

Gallery store: similar_to filter field + loadSimilar() (ranked, hasMore=false,
no timeline); applyFilterFromQuery routes similar-mode to /similar with the
scope filters composed; cloneFilter/filterToQuery carry similar_to. Filter bar:
clearable 'Similar to #id' chip, sort hidden in similar-mode; timeline sidebar
hidden too.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/views/GalleryView.vue

@@ -16,7 +16,10 @@
         <EmptyState v-if="store.isEmpty" />
         <GalleryGrid v-else @open="openImage" />
       </div>
-      <TimelineSidebar v-if="store.images.length > 0" class="fc-gallery-layout__sidebar" />
+      <TimelineSidebar
+        v-if="store.images.length > 0 && store.filter.similar_to == null"
+        class="fc-gallery-layout__sidebar"
+      />
     </div>
 
     <BulkEditorPanel />