feat(flutter): #396 — full-screen now-playing polish

Three of the four locked items (1, 2, 4); item 6 (swipe-tabs) stays
deferred until server-side lyrics ingestion exists.

1. Dominant-color gradient backdrop. New album_color_extractor.dart
   wraps the existing AlbumCoverCache: extracts the dominant color
   via PaletteGenerator over the local file, caches in-memory keyed
   by album_id. Top 55% of the screen carries the color (0.55 alpha
   → fs.obsidian) so controls below stay legible. AnimatedContainer
   tweens the gradient across track changes.

2. Hero transition for cover art (mini bar → full screen). Stable
   kPlayerCoverHeroTag (not media.id keyed) so the transition works
   regardless of what's playing and isn't racy if media swaps mid-tap.
   flightShuttleBuilder renders the destination's Hero widget for the
   whole flight, which reads as a clean grow rather than a swap.

4. Crossfade on track change. AnimatedSwitcher around the album art,
   title, and artist+album text block, all keyed by media.id so the
   switcher fades between old and new on each track-change rebuild.
   Pairs with the AnimatedContainer gradient so the whole "what's
   playing" zone changes in lockstep.

palette_generator: ^0.3.3 added.

For #396 / #356 umbrella.

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

flutter_client/pubspec.lock

@@ -696,6 +696,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "3.2.1"
+  palette_generator:
+    dependency: "direct main"
+    description:
+      name: palette_generator
+      sha256: "4420f7ccc3f0a4a906144e73f8b6267cd940b64f57a7262e95cb8cec3a8ae0ed"
+      url: "https://pub.dev"
+    source: hosted
+    version: "0.3.3+7"
   path:
     dependency: transitive
     description: