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/services/api.ts

@@ -188,6 +188,23 @@ export const heaps = {
     })
     return response.data
   },
+
+  /** Convert a heap into a folder by moving (or copying) every member
+   *  photo into the target directory. */
+  convert: async (
+    heapId: string,
+    body: { target_id: string; mode: 'move' | 'copy'; delete_heap: boolean }
+  ) => {
+    const response = await api.post(`/heaps/${heapId}/convert`, body)
+    return response.data as {
+      status: string
+      mode: 'move' | 'copy'
+      moved: number
+      copied: number
+      errors: Array<{ id: string; error: string }>
+      heap_deleted: boolean
+    }
+  },
 }
 
 // Tags API