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/gallery/GalleryFilterBar.vue

@@ -38,6 +38,12 @@
         prepend-icon="mdi-account"
         @click:close="clearArtist"
       >{{ store.artistLabel || `Artist #${store.filter.artist_id}` }}</v-chip>
+      <v-chip
+        v-if="store.filter.similar_to"
+        size="small" closable color="accent" variant="tonal"
+        prepend-icon="mdi-image-multiple"
+        @click:close="clearSimilar"
+      >Similar to #{{ store.filter.similar_to }}</v-chip>
     </div>
 
     <v-spacer />
@@ -52,7 +58,9 @@
       <v-btn value="video" size="small">Videos</v-btn>
     </v-btn-toggle>
 
+    <!-- Sort is meaningless in similar-mode (results are distance-ranked). -->
     <v-select
+      v-if="!store.filter.similar_to"
       :model-value="store.filter.sort"
       :items="SORTS"
       density="compact" hide-details variant="outlined"
@@ -114,6 +122,7 @@ const hasActiveFilters = computed(() =>
   store.filter.artist_id != null ||
   store.filter.media_type != null ||
   store.filter.sort !== 'newest' ||
+  store.filter.similar_to != null ||
   hasRefineFilters.value
 )
 
@@ -190,6 +199,7 @@ function clearArtist() {
   store.noteArtistLabel(null)
   pushFilter((n) => { n.artist_id = null })
 }
+function clearSimilar() { pushFilter((n) => { n.similar_to = null }) }
 function setMedia(m) { pushFilter((n) => { n.media_type = m }) }
 function setSort(s) { pushFilter((n) => { n.sort = s }) }
 function clearAll() { router.push({ name: 'gallery', query: {} }) }