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>

backend/app/schemas/library.py

@@ -55,6 +55,7 @@ class LibraryCreate(BaseModel):
 class LibraryUpdate(BaseModel):
     name: str | None = None
     folders: list[dict] | None = None
+    media_type: str | None = None
     settings: dict | None = None