fix(flutter): five flutter analyze --fatal-infos issues from first CI run

1. lib/app.dart + lib/shared/routing.dart — buildRouter takes a Ref but was
   being called from a ConsumerWidget's build() with a WidgetRef. Add a
   routerProvider; the widget watches it instead of constructing the
   router from its own ref. Real bug — would have crashed compile in
   slice 1, just never compiled until CI ran.

2. lib/player/audio_handler.dart — override of AudioHandler.seek used
   `p` for the Duration; rename to `position` to match the base class
   (avoid_renaming_method_parameters lint).

3. lib/theme/theme_extension.dart — fromTokens() returned a non-const
   constructor; all inputs are const so make the call const too
   (prefer_const_constructors).

4. test/library/home_screen_test.dart — same const-constructor lint on
   the HomeData test fixture.

5. test/smoke_test.dart — drop the unused
   `import 'package:flutter/material.dart'`.

These are exactly the kind of issues the no-in-task-tests rule shifted
to CI; this is the first run that actually exercises the analyzer
against the slice 1 code.

flutter_client/lib/app.dart

@@ -9,7 +9,7 @@ class MinstrelApp extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final router = buildRouter(ref);
+    final router = ref.watch(routerProvider);
     return MaterialApp.router(
       title: 'Minstrel',
       theme: buildThemeData(),