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/stores/gallery.js

@@ -27,6 +27,9 @@ export const useGalleryStore = defineStore('gallery', () => {
     // Phase-2 faceted refine params.
     platform: null, untagged: false, no_artist: false,
     date_from: null, date_to: null,
+    // Phase-3 visual similarity: when set, the gallery is in "similar mode" —
+    // ranked by cosine distance to this image, bounded top-N, no cursor.
+    similar_to: null,
   })
 
   // Live facet counts for the refine panel; fetched on-demand (panel open +
@@ -84,6 +87,39 @@ export const useGalleryStore = defineStore('gallery', () => {
     }
   }
 
+  // Visual "more like this": ranked top-N by cosine distance, scope filters
+  // composed (AND). No cursor / no timeline — bounded result set.
+  async function loadSimilar() {
+    inflight.cancel()
+    images.value = []
+    dateGroups.value = []
+    nextCursor.value = null
+    timelineBuckets.value = []
+    loading.value = true
+    error.value = null
+    const t = inflight.claim()
+    try {
+      const f = filter.value
+      const params = { similar_to: f.similar_to, limit: 100 }
+      if (f.tag_ids.length) params.tag_id = f.tag_ids.join(',')
+      if (f.artist_id) params.artist_id = f.artist_id
+      if (f.media_type) params.media = f.media_type
+      if (f.platform) params.platform = f.platform
+      if (f.untagged) params.untagged = '1'
+      if (f.no_artist) params.no_artist = '1'
+      if (f.date_from) params.date_from = f.date_from
+      if (f.date_to) params.date_to = f.date_to
+      const body = await api.get('/api/gallery/similar', { params })
+      if (!t.isCurrent()) return
+      images.value = body.images
+      // ranked + bounded → no next page (nextCursor stays null → hasMore false)
+    } catch (e) {
+      error.value = e.message
+    } finally {
+      if (t.isCurrent()) loading.value = false
+    }
+  }
+
   async function jumpTo(year, month) {
     // Rapid timeline-jump clicks need the same race guard as
     // loadMore — first jump's late body could clobber the second
@@ -150,9 +186,15 @@ export const useGalleryStore = defineStore('gallery', () => {
       no_artist: _truthy(q.no_artist),
       date_from: _parseDate(q.date_from),
       date_to: _parseDate(q.date_to),
+      similar_to: _toId(q.similar_to),
+    }
+    if (filter.value.similar_to) {
+      // Similar mode: ranked, no timeline scroll.
+      await loadSimilar()
+    } else {
+      await loadInitial()
+      await loadTimeline()
     }
-    await loadInitial()
-    await loadTimeline()
     _resolveLabels()
   }
 
@@ -200,7 +242,7 @@ export const useGalleryStore = defineStore('gallery', () => {
     images, dateGroups, hasMore, isEmpty, loading, error,
     filter, tagLabels, artistLabel, timelineBuckets, timelineLoading,
     facets, facetsLoading,
-    loadInitial, loadMore, loadTimeline, jumpTo, loadFacets,
+    loadInitial, loadMore, loadTimeline, jumpTo, loadFacets, loadSimilar,
     applyFilterFromQuery, noteTagLabel, noteArtistLabel,
   }
 })
@@ -213,6 +255,7 @@ export function cloneFilter(f) {
     tag_ids: [...f.tag_ids], artist_id: f.artist_id, media_type: f.media_type,
     sort: f.sort, platform: f.platform, untagged: f.untagged,
     no_artist: f.no_artist, date_from: f.date_from, date_to: f.date_to,
+    similar_to: f.similar_to,
   }
 }
 
@@ -227,6 +270,7 @@ export function filterToQuery(f) {
   if (f.no_artist) q.no_artist = '1'
   if (f.date_from) q.date_from = f.date_from
   if (f.date_to) q.date_to = f.date_to
+  if (f.similar_to) q.similar_to = String(f.similar_to)
   return q
 }