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/heaps/HeapsPanel.tsx

@@ -6,14 +6,16 @@ import {
   X,
   ChevronDown,
   ChevronRight,
+  FolderOutput,
 } from 'lucide-react'
 import clsx from 'clsx'
 import { useMutation, useQueryClient } from '@tanstack/react-query'
 import { useHeapsQuery, HEAPS_QUERY_KEY } from '../../hooks/useHeapsQuery'
-import { heaps as heapsApi } from '../../services/api'
+import { heaps as heapsApi, type Heap } from '../../services/api'
 import { useFilterStore } from '../../store/filterStore'
 import { toast } from '../ToastContainer'
 import { PHOTO_DRAG_MIME } from '../timeline/PhotoThumbnail'
+import { HeapConvertDialog } from './HeapConvertDialog'
 
 /**
  * Heaps panel for the left sidebar. Renders the list of heaps with the
@@ -37,6 +39,7 @@ export function HeapsPanel() {
   // Which heap row is currently being hovered with a drag — used to render
   // the drop highlight ring. Only one heap can be the target at a time.
   const [dropTargetId, setDropTargetId] = useState<string | null>(null)
+  const [convertingHeap, setConvertingHeap] = useState<Heap | null>(null)
 
   const invalidate = () => {
     queryClient.invalidateQueries({ queryKey: HEAPS_QUERY_KEY })
@@ -277,6 +280,16 @@ export function HeapsPanel() {
                 >
                   <Target className="h-3 w-3" />
                 </button>
+                <button
+                  onClick={(e) => {
+                    e.stopPropagation()
+                    setConvertingHeap(heap)
+                  }}
+                  className="invisible rounded p-0.5 text-text-muted hover:bg-surface-offset hover:text-text group-hover:visible"
+                  title="Convert to folder…"
+                >
+                  <FolderOutput className="h-3 w-3" />
+                </button>
                 <button
                   onClick={(e) => {
                     e.stopPropagation()
@@ -294,6 +307,11 @@ export function HeapsPanel() {
           })}
         </div>
       )}
+
+      <HeapConvertDialog
+        heap={convertingHeap}
+        onClose={() => setConvertingHeap(null)}
+      />
     </div>
   )
 }