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>

backend/app/routers/heaps.py

@@ -39,6 +39,10 @@ class HeapConvertBody(BaseModel):
     target_id: str  # folder id OR source root id
     mode: Literal['move', 'copy'] = 'move'
     delete_heap: bool = False
+    # Optional subfolder name to create inside the target. If provided, the
+    # actual destination is target_dir/subfolder_name (created if missing).
+    # Path separators and dot-segments are rejected.
+    subfolder_name: Optional[str] = None
 
 
 # ── Endpoints ─────────────────────────────────────────────────────────────
@@ -217,24 +221,53 @@ async def convert_heap_to_folder(
     source_root = sr_check.scalar_one_or_none()
 
     if source_root is not None:
-        target_dir = source_root.path
-        from app.tasks.scan import get_or_create_folder
-        target_folder = await get_or_create_folder(db, target_dir, source_root.id)
+        parent_dir = source_root.path
+        parent_source_root_id = source_root.id
     else:
         folder_check = await db.execute(
             select(Folder).where(Folder.id == body.target_id)
         )
-        target_folder = folder_check.scalar_one_or_none()
-        if target_folder is None:
+        parent_folder = folder_check.scalar_one_or_none()
+        if parent_folder is None:
             raise HTTPException(status_code=404, detail="Target folder not found")
-        target_dir = target_folder.path
+        parent_dir = parent_folder.path
+        parent_source_root_id = parent_folder.source_root_id
 
-    if not os.path.isdir(target_dir):
+    if not os.path.isdir(parent_dir):
         raise HTTPException(
             status_code=400,
-            detail=f"Target directory does not exist: {target_dir}",
+            detail=f"Target parent does not exist: {parent_dir}",
         )
 
+    # Resolve target_dir, creating an optional subfolder if requested.
+    if body.subfolder_name is not None:
+        sub = body.subfolder_name.strip()
+        if not sub:
+            raise HTTPException(status_code=400, detail="Subfolder name cannot be empty")
+        if '/' in sub or '\\' in sub or sub in ('.', '..'):
+            raise HTTPException(status_code=400, detail="Invalid subfolder name")
+        target_dir = os.path.join(parent_dir, sub)
+        if not os.path.exists(target_dir):
+            try:
+                os.makedirs(target_dir)
+            except OSError as e:
+                raise HTTPException(
+                    status_code=500,
+                    detail=f"Failed to create subfolder: {e}",
+                )
+        elif not os.path.isdir(target_dir):
+            raise HTTPException(
+                status_code=400,
+                detail=f"{target_dir} exists but is not a directory",
+            )
+    else:
+        target_dir = parent_dir
+
+    # Ensure a Folder row for the target, reusing the scanner helper so
+    # path normalization + dedupe stay consistent.
+    from app.tasks.scan import get_or_create_folder
+    target_folder = await get_or_create_folder(db, target_dir, parent_source_root_id)
+
     # Fetch the heap's photos via the join table.
     photo_result = await db.execute(
         select(Photo)