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/routers/libraries.py

@@ -172,6 +172,8 @@ async def update_library(
             {"id": f.get("id", str(uuid.uuid4())), "fullPath": f.get("fullPath", f.get("full_path", ""))}
             for f in body.folders
         ]
+    if body.media_type is not None:
+        lib.media_type = body.media_type
     if body.settings is not None:
         lib.settings = {**(lib.settings or {}), **body.settings}