fix: heap convert dialog supports nested subfolders + visiblePhotoIds loop guard

- HeapConvertDialog: switch the target picker from sourceFolders.list
  (top-level source roots only) to useFolderTreeQuery, flattened
  depth-first into a list with depth info. Each option is indented
  with non-breaking spaces so nested subfolders read as a tree in
  the native dropdown. Backend already accepts any Folder id, so no
  server change needed.
- photoStore.setVisiblePhotoIds: short-circuit when the new id list
  matches the existing one element-for-element. Avoids feedback loops
  if a publisher fires from an effect on a render where the contents
  haven't actually changed (which was triggering React error #185).
- Timeline: pull setVisiblePhotoIds via a focused selector instead of
  the wholesale destructure so the publisher subscription doesn't
  re-render Timeline on unrelated photo store changes.

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

frontend/src/components/timeline/Timeline.tsx

@@ -172,8 +172,10 @@ export function Timeline() {
     togglePhotoSelection,
     clearSelection,
     openPreview,
-    setVisiblePhotoIds,
   } = usePhotoStore()
+  // Pulled via a focused selector so the publisher subscription doesn't
+  // re-render Timeline on every unrelated photo store change.
+  const setVisiblePhotoIds = usePhotoStore((s) => s.setVisiblePhotoIds)
 
   const sortBy = useFilterStore((s) => s.sortBy)
   const groupBy = useFilterStore((s) => s.groupBy)