feat: heap convert can create a subfolder under the target

The previous /heaps/{id}/convert dropped photos directly into a chosen
source root, which is rarely what you want — Lightroom-style behaviour
is "make a folder named after the collection inside the library".
Now the dialog lets you do that.

Backend
- HeapConvertBody gains an optional subfolder_name field. Path
  separators and dot-segments are rejected. When set, the handler
  joins it onto the resolved parent_dir, mkdir's it if missing, and
  uses the resulting path as the move/copy destination. Otherwise
  the parent_dir itself is used (unchanged behaviour).
- The Folder DB row for the destination is created via the existing
  scanner get_or_create_folder helper so dedupe + path normalization
  stay consistent across the codebase.
- The target source root id is propagated through both the source-
  root and folder branches so the new Folder row is correctly
  parented when subfolder_name is set on a folder target too.

Frontend
- HeapConvertDialog grows a "Subfolder name" input that prefills
  with the heap name when the dialog opens. Trimmed empty value
  drops directly into the parent. A live hint below the input
  shows exactly which path will be created (or that the parent
  will be used).
- api.ts heaps.convert() signature accepts an optional
  subfolder_name field; the dialog sends it via mutationFn.

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

frontend/src/components/heaps/HeapConvertDialog.tsx

@@ -21,6 +21,7 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
   const [targetId, setTargetId] = useState('')
   const [mode, setMode] = useState<'move' | 'copy'>('move')
   const [deleteHeap, setDeleteHeap] = useState(false)
+  const [subfolderName, setSubfolderName] = useState('')
 
   const { data: foldersData } = useQuery({
     queryKey: ['folders'],
@@ -36,12 +37,15 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
     }
   }, [folders, targetId])
 
-  // Reset state on close.
+  // Reset state on close, prefill subfolder name when opened.
   useEffect(() => {
-    if (!heap) {
+    if (heap) {
+      setSubfolderName(heap.name)
+    } else {
       setTargetId('')
       setMode('move')
       setDeleteHeap(false)
+      setSubfolderName('')
     }
   }, [heap])
 
@@ -51,6 +55,9 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
         target_id: targetId,
         mode,
         delete_heap: deleteHeap,
+        // Empty subfolder = drop directly into the parent. Trim and only
+        // send if the user kept it populated.
+        subfolder_name: subfolderName.trim() || null,
       }),
     onSuccess: (data) => {
       const total = (data.moved ?? 0) + (data.copied ?? 0)
@@ -118,6 +125,25 @@ export function HeapConvertDialog({ heap, onClose }: HeapConvertDialogProps) {
           )}
         </div>
 
+        {/* Subfolder name */}
+        <div className="mb-4">
+          <label className="mb-1 block text-xs text-text-muted">
+            Subfolder name
+          </label>
+          <input
+            type="text"
+            value={subfolderName}
+            onChange={(e) => setSubfolderName(e.target.value)}
+            placeholder="(none — use parent directly)"
+            className="w-full rounded border border-border bg-bg px-2 py-1.5 text-sm text-text placeholder-text-faint focus:border-primary focus:outline-none"
+          />
+          <p className="mt-1 text-xs text-text-faint">
+            {subfolderName.trim() && targetFolder
+              ? `Will create ${targetFolder.path}/${subfolderName.trim()} if missing.`
+              : 'Photos go directly into the parent folder.'}
+          </p>
+        </div>
+
         {/* Mode toggle */}
         <div className="mb-4">
           <label className="mb-1 block text-xs text-text-muted">Mode</label>