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/components/common/FileBrowser.tsx

@@ -49,6 +49,16 @@ export default function FileBrowser({ initialPath = '/', onSelect, onClose }: Pr
           </button>
         </div>
 
+        {/* Quick access */}
+        <div className="flex gap-1 px-3 py-2 border-b border-white/10 flex-shrink-0 overflow-x-auto">
+          {['/audiofiles', '/data', '/media', '/'].map((p) => (
+            <button key={p} onClick={() => load(p)}
+              className={`flex-shrink-0 px-2 py-1 rounded text-xs transition-colors ${path === p ? 'bg-primary/20 text-primary' : 'text-gray-500 hover:text-white hover:bg-white/5'}`}>
+              {p}
+            </button>
+          ))}
+        </div>
+
         {/* Current path */}
         <div className="px-4 py-2 bg-white/5 border-b border-white/10 flex items-center gap-2 flex-shrink-0">
           {parent && (