feat(flutter): user-facing requests view + cancel (#356)

Closes the cleanest remaining #356 gap — Discover submits Lidarr
requests but had no surface for users to view or cancel their own.
Admin had it; user didn't.

- RequestsApi at lib/api/endpoints/requests.dart — listMine() +
  cancel(id). Same /api/requests endpoint the web /requests page
  uses; identical AdminRequest wire shape so the existing model
  is reused (just augmented with matched_*_id fields for the
  "Listen" CTA on completed rows).
- MyRequestsController mirrors the web's auto-poll (#369 piece
  already shipped server-side / web-side): 12s refresh while any
  row is mid-ingest (status='approved'), stops on settle. Riverpod
  Timer in build() that's cancelled in onDispose.
- RequestsScreen with kind/status pills, ingest progress copy,
  Cancel (with confirm dialog) and Listen CTAs per row state.
- Route /requests under the shell. Entry point in settings between
  Profile and Appearance — "My requests".
- Widget tests cover empty / pending / completed / rejected states +
  the Cancel-confirm dialog.

Out of scope this slice: Discover-screen "View your requests" CTA
after submitting a new request. Reasonable follow-up but doesn't
block the management loop.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/shared/routing.dart

@@ -15,6 +15,7 @@ import '../player/player_bar.dart';
 import '../player/queue_screen.dart';
 import '../playlists/playlist_detail_screen.dart';
 import '../playlists/playlists_list_screen.dart';
+import '../requests/requests_screen.dart';
 import '../search/search_screen.dart';
 import '../settings/settings_screen.dart';
 import '../admin/admin_landing_screen.dart';
@@ -75,6 +76,7 @@ GoRouter buildRouter(Ref ref) {
             path: '/playlists/:id',
             builder: (_, s) => PlaylistDetailScreen(id: s.pathParameters['id']!),
           ),
+          GoRoute(path: '/requests',         builder: (_, __) => const RequestsScreen()),
           GoRoute(path: '/admin',            builder: (_, __) => const AdminLandingScreen()),
           GoRoute(path: '/admin/requests',   builder: (_, __) => const AdminRequestsScreen()),
           GoRoute(path: '/admin/quarantine', builder: (_, __) => const AdminQuarantineScreen()),