feat(tablet): navigation rail, landscape now-playing, clamped track lists

- shell switches to a NavigationRail at >=600dp (same destinations,
  badges included); the bottom bar and its blur stay phone-only while
  the mini player remains anchored bottom in both modes
- now-playing gains a two-pane landscape layout (artwork left,
  metadata/controls right) built from the same widget pieces
- track lists on album/playlist/library-folder screens and search
  result sections center at 720dp via wideListInset instead of
  stretching across the full tablet width; explore carousels stay
  full-bleed intentionally

lib/utils/adaptive_layout.dart

@@ -0,0 +1,9 @@
+import 'package:flutter/widgets.dart';
+
+/// Horizontal inset that centers full-width list content at [contentMaxWidth]
+/// on tablets/landscape; zero on phones, so rows stop stretching across the
+/// whole screen.
+double wideListInset(BuildContext context, {double contentMaxWidth = 720}) {
+  final width = MediaQuery.sizeOf(context).width;
+  return width > contentMaxWidth ? (width - contentMaxWidth) / 2 : 0;
+}