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/components/modal/ImageViewer.vue

@@ -51,6 +51,9 @@
         <aside v-if="modal.current" class="fc-viewer__side">
           <ProvenancePanel />
           <TagPanel />
+          <!-- Non-blocking: fetches its own similar set after the modal is up;
+               collapses silently if empty/slow/failed (see RelatedStrip). -->
+          <RelatedStrip />
         </aside>
       </div>
     </div>
@@ -65,6 +68,7 @@ import ImageCanvas from './ImageCanvas.vue'
 import VideoCanvas from './VideoCanvas.vue'
 import TagPanel from './TagPanel.vue'
 import ProvenancePanel from './ProvenancePanel.vue'
+import RelatedStrip from './RelatedStrip.vue'
 
 const emit = defineEmits(['close'])