fix(ui): make screen backgrounds full-bleed instead of content-sized

The gradient Container in the playlist selection and admin screens sized
itself to its content rather than the viewport: under the loose body
constraints, SingleChildScrollView and Wrap both shrink-wrap, so the
gradient was painted only across a 436 px band (48 padding + 340 card +
48 padding) with the rest of the screen falling through to bare black.
BoxConstraints.expand() forces full coverage.

Also:
- both screens now use AppColors.backgroundGradient. The previous ramp
  ran surfaceContainerLowest -> background -> baseLevel0, i.e. dark to
  lighter to dark, which put a pale band across the middle
- the playlist card was a 3% cream translucent plate, so it took on
  whatever sat behind it instead of resting on it; it is now an opaque
  surface (surfaceContainerLow, surfaceContainerHigh on hover) with the
  lift() shadow at rest and the ember focus halo on hover
- Scaffold backgroundColor pinned to baseLevel0 so route transitions
  never flash pure black

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

lib/features/admin/screens/admin_panel.dart

@@ -24,16 +24,13 @@ class AdminPanel extends StatelessWidget {
   Widget build(BuildContext context) {
     return Scaffold(
       extendBodyBehindAppBar: true,
+      backgroundColor: AppColors.baseLevel0,
       body: Container(
+        // Meme precaution que sur l'ecran de selection de playlist : sans
+        // contrainte d'expansion, le degrade ne couvre que le contenu.
+        constraints: const BoxConstraints.expand(),
         decoration: const BoxDecoration(
-          gradient: LinearGradient(
-            begin: Alignment.topCenter,
-            end: Alignment.bottomCenter,
-            colors: [
-              AppColors.surfaceVariant,
-              AppColors.background,
-            ],
-          ),
+          gradient: AppColors.backgroundGradient,
         ),
         child: const AdminContent(),
       ),