fix(#105): ensure folders are nested inside user home folder

Root cause: when window.app.currentPath was empty/falsy (due to timing,
page state reset, or initialization), the frontend sent parent_id: null.
The backend then created folders at the storage root instead of inside
the user's home folder.

Backend fix (folder_handler.rs):
- Added AuthUser extractor to create_folder handler
- When parent_id is None, auto-resolves the user's home folder
  ('My Folder - {username}') as the parent folder
- Folders are now always created inside the user's directory tree

Frontend fix (fileOperations.js):
- Changed parent_id fallback from null to window.app.userHomeFolderId
- Prevents sending null parent_id even if currentPath is reset

Search in subfolders: no fix needed — search_recursive() already
traverses the filesystem correctly; it was only failing because folders
were physically flat instead of nested.

Bumps service worker cache to v12.

static/js/fileOperations.js

@@ -342,7 +342,7 @@ const fileOps = {
                 },
                 body: JSON.stringify({
                     name: name,
-                    parent_id: window.app.currentPath || null
+                    parent_id: window.app.currentPath || window.app.userHomeFolderId || null
                 })
             });