feat: heap convert-to-folder + surface exact-duplicate detection

Two related polish items.

1. Heap convert to folder
   Closes a long-standing TODO from spec §6.10.
   - Backend: POST /heaps/{id}/convert with body
       { target_id, mode: 'move'|'copy', delete_heap: bool }
     target_id resolves either as a Folder id or a SourceRoot id (same
     convention as /photos/move). For each member photo, dispatches
     either shutil.move + photo.folder_id update, or shutil.copy2 +
     a new is_duplicate=true Photo row with all metadata copied. Name
     collisions on copy use the same " (copy N)" suffix scheme as
     /photos/copy. The heap row is optionally deleted on success.
     Per-photo failures are collected into the response instead of
     aborting the batch.
   - Frontend: new HeapConvertDialog with a target-folder dropdown
     (currently from sourceFolders.list, sub-folder picking is a
     follow-up), move/copy radio, and a "delete heap" checkbox.
     HeapsPanel rows get a hover FolderOutput button that opens it.
     Toast on success names the verb + count and notes whether the
     heap was deleted; invalidates heaps + photos + folders queries.

2. Surface exact-duplicate detection
   The scanner already sets Photo.is_duplicate=true when a SHA-256
   match is found, but nothing surfaced it. Now:
   - Backend list_photos accepts an optional is_duplicate query
     param so the frontend can filter duplicates-only views.
   - filterStore gains a duplicates: boolean field with setter, URL
     sync (?duplicates=true), filtersToParams entry, and a
     hasActiveFilters check.
   - LeftSidebar gets a new "Duplicates" library node (Copy icon)
     that clearAllFilters() + setDuplicates(true). isItemActive
     follows the filter so the highlight stays in sync after
     external filter changes.
   - PhotoThumbnail renders a small dark badge with the Copy icon
     bottom-right when photo.is_duplicate. Sits next to the existing
     basket / discard badges so the user can spot duplicates at a
     glance.
   - Photo TS type adds is_duplicate.

Perceptual-hash duplicate detection (re-encoded / resized matches)
is intentionally a follow-up — needs an imagehash dep, a phash
column, a backfill job, and similarity-search endpoint with
hamming-distance grouping. This commit only surfaces what the
scanner already finds via byte-level SHA-256 comparison.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/hooks/useFilterUrlSync.ts

@@ -77,6 +77,8 @@ function parseUrl(): Partial<FilterState> {
     if (ids.length > 0) out.tagIds = ids
   }
 
+  if (sp.get('duplicates') === 'true') out.duplicates = true
+
   const sortBy = sp.get('sort')
   if (sortBy && ALLOWED_SORT_FIELDS.includes(sortBy as SortField)) {
     out.sortBy = sortBy as SortField
@@ -102,6 +104,7 @@ function writeUrl(f: FilterState) {
   if (f.heapId) sp.set('heap_id', f.heapId)
   if (f.folderId) sp.set('folder_id', f.folderId)
   if (f.tagIds.length > 0) sp.set('tag_ids', f.tagIds.join(','))
+  if (f.duplicates) sp.set('duplicates', 'true')
   if (f.sortBy !== 'taken_at') sp.set('sort', f.sortBy)
   if (f.sortOrder !== 'desc') sp.set('order', f.sortOrder)
 

frontend/src/hooks/usePhotosQuery.ts

@@ -21,6 +21,7 @@ export function usePhotosQuery() {
   const heapId = useFilterStore((s) => s.heapId)
   const folderId = useFilterStore((s) => s.folderId)
   const tagIds = useFilterStore((s) => s.tagIds)
+  const duplicates = useFilterStore((s) => s.duplicates)
   const sortBy = useFilterStore((s) => s.sortBy)
   const sortOrder = useFilterStore((s) => s.sortOrder)
 
@@ -37,10 +38,11 @@ export function usePhotosQuery() {
         heapId,
         folderId,
         tagIds,
+        duplicates,
         sortBy,
         sortOrder,
       }),
-    [q, dateFrom, dateTo, mediaTypes, ratingMin, colorLabel, flag, heapId, folderId, tagIds, sortBy, sortOrder]
+    [q, dateFrom, dateTo, mediaTypes, ratingMin, colorLabel, flag, heapId, folderId, tagIds, duplicates, sortBy, sortOrder]
   )
 
   return useQuery({