Fix sidebar, add library editing, improve file browser

- Sidebar: show Podcasts section only when podcast libraries exist;
  show hint text when no libraries are configured yet
- Admin: extract LibraryForm component, add edit (Pencil) button per
  library with inline form and PATCH support
- Backend: add media_type to LibraryUpdate schema and PATCH handler
- FileBrowser: add quick-access shortcuts (/audiofiles /data /media /),
  show all files+dirs so users can confirm correct folder

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/libraries.ts

@@ -17,5 +17,8 @@ export const scanLibrary = (libraryId: string) =>
 export const createLibrary = (data: object) =>
   api.post('/api/libraries', data).then((r) => r.data)
 
+export const updateLibrary = (id: string, data: object) =>
+  api.patch(`/api/libraries/${id}`, data).then((r) => r.data)
+
 export const deleteLibrary = (id: string) =>
   api.delete(`/api/libraries/${id}`).then((r) => r.data)