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/components/layout/LeftSidebar.tsx

@@ -9,6 +9,7 @@ import {
   MoreHorizontal,
   HardDrive,
   RefreshCw,
+  Copy,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { sourceFolders, library, photos as photosApi } from '../../services/api'
@@ -41,7 +42,9 @@ export function LeftSidebar() {
   const setRatingMin = useFilterStore((s) => s.setRatingMin)
   const setFlag = useFilterStore((s) => s.setFlag)
   const setFolderId = useFilterStore((s) => s.setFolderId)
+  const setDuplicates = useFilterStore((s) => s.setDuplicates)
   const filterFolderId = useFilterStore((s) => s.folderId)
+  const filterDuplicates = useFilterStore((s) => s.duplicates)
   const [dropTargetId, setDropTargetId] = useState<string | null>(null)
 
   // Bulk discard mutation for the drag-onto-Discarded interaction.
@@ -129,6 +132,10 @@ export function LeftSidebar() {
         clearAllFilters()
         setFlag('discarded')
         break
+      case 'duplicates':
+        clearAllFilters()
+        setDuplicates(true)
+        break
       default:
         if (id.startsWith('folder-')) {
           // Folder rows: filter to that folder, clear other filters that
@@ -199,6 +206,7 @@ export function LeftSidebar() {
       children: [
         { id: 'all-photos', label: 'All Photos', icon: <Image className="h-4 w-4" />, count: 0 },
         { id: 'rated', label: 'Rated', icon: <Star className="h-4 w-4" />, count: 0 },
+        { id: 'duplicates', label: 'Duplicates', icon: <Copy className="h-4 w-4" />, count: 0 },
         { id: 'discarded', label: 'Discarded', icon: <Trash2 className="h-4 w-4" />, count: 0 },
       ],
     },
@@ -227,6 +235,9 @@ export function LeftSidebar() {
     if (id === 'all-photos') {
       return filterFolderId === null && selectedItem === 'all-photos'
     }
+    if (id === 'duplicates') {
+      return filterDuplicates
+    }
     return selectedItem === id
   }