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/test/library/home_screen_test.dart

@@ -11,7 +11,7 @@ import 'package:minstrel/theme/theme_data.dart';
 
 void main() {
   testWidgets('home renders sections with rows', (tester) async {
-    final fixture = HomeData(
+    const fixture = HomeData(
       recentlyAddedAlbums: const [
         AlbumRef(id: 'a', title: 'Geogaddi', artistId: 'x', artistName: 'BoC'),
       ],

flutter_client/test/smoke_test.dart

@@ -1,4 +1,3 @@
-import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:flutter_test/flutter_test.dart';