fix(flutter): analyzer issues from #396 — unused import + AsyncValue API

Two issues caught by flutter analyze --fatal-infos:

- dart:ui import in album_color_extractor.dart was redundant because
  flutter/painting re-exports the Color it provides. Dropped.
- valueOrNull isn't on AsyncValue in this Riverpod version (the
  AsyncValue<Color?> nesting may also have confused the resolver).
  Switched to asData?.value which always returns the wrapped value
  on AsyncData and null on Loading/Error.

(palette_generator's "discontinued" warning is non-fatal informational
in pub; CI didn't fail on it. The package still works; alternative
swaps deferred until it actually breaks.)

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

flutter_client/lib/player/album_color_extractor.dart

@@ -13,7 +13,6 @@
 // obsidian color for the gradient when null.
 
 import 'dart:io';
-import 'dart:ui';
 
 import 'package:flutter/painting.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';

flutter_client/lib/player/now_playing_screen.dart

@@ -92,7 +92,7 @@ class _NowPlayingScreenState extends ConsumerState<NowPlayingScreen> {
     // before the polish landed. Tweens to the new color on track change
     // via AnimatedContainer.
     final dominantAsync = ref.watch(albumColorProvider(albumId));
-    final dominant = dominantAsync.valueOrNull ?? fs.obsidian;
+    final dominant = dominantAsync.asData?.value ?? fs.obsidian;
     // 0.55 alpha keeps the color present without overwhelming contrast
     // on the title / artist text below. The lower half of the screen
     // stays obsidian for control legibility.