feat(web): add /settings page with ListenBrainz section

Adds api.put helper, ListenBrainz API module with query/mutation
helpers, a /settings route with token + enable/disable UI, and 5
tests. Adds Settings to the Shell nav.

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

web/src/lib/api/client.ts

@@ -45,6 +45,8 @@ export const api = {
   get: <T>(path: string): Promise<T> => apiFetch(path) as Promise<T>,
   post: <T>(path: string, body: unknown): Promise<T> =>
     apiFetch(path, { method: 'POST', body: JSON.stringify(body) }) as Promise<T>,
+  put: <T>(path: string, body: unknown): Promise<T> =>
+    apiFetch(path, { method: 'PUT', body: JSON.stringify(body) }) as Promise<T>,
   del: (path: string): Promise<null> =>
     apiFetch(path, { method: 'DELETE' }) as Promise<null>
 };