perf: cache ThemeColors across frames; theme-tint leaf particles

Fix 1: Stop allocating ThemeColors on every animation frame by caching it in
_colors field and updating only when theme changes via didUpdateWidget.

Fix 2: Leaf particles now follow the season accent color instead of hardcoded
orange, maintaining visual consistency with theme changes.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/widgets/season_background.dart

@@ -28,12 +28,20 @@ class _SeasonBackgroundState extends State<SeasonBackground>
     duration: const Duration(seconds: 18),
   );
 
+  late ThemeColors _colors = ThemeColors(widget.theme);
+
   @override
   void initState() {
     super.initState();
     if (!debugDisableLoopingAnimations) _drift.repeat();
   }
 
+  @override
+  void didUpdateWidget(SeasonBackground old) {
+    super.didUpdateWidget(old);
+    if (old.theme != widget.theme) _colors = ThemeColors(widget.theme);
+  }
+
   @override
   void dispose() {
     _drift.dispose();
@@ -42,13 +50,12 @@ class _SeasonBackgroundState extends State<SeasonBackground>
 
   @override
   Widget build(BuildContext context) {
-    final colors = ThemeColors(widget.theme);
     return RepaintBoundary(
       child: AnimatedBuilder(
         animation: _drift,
         builder: (context, _) => CustomPaint(
           size: Size.infinite,
-          painter: _AmbiencePainter(colors: colors, t: _drift.value),
+          painter: _AmbiencePainter(colors: _colors, t: _drift.value),
         ),
       ),
     );
@@ -110,7 +117,7 @@ class _AmbiencePainter extends CustomPainter {
         paint.color = Colors.white.withValues(alpha: 0.35);
         canvas.drawCircle(Offset.zero, s * 0.4, paint);
       case 'leaves':
-        paint.color = const Color(0xFFE8945A).withValues(alpha: 0.35);
+        paint.color = colors.accent.withValues(alpha: 0.35);
         canvas.drawOval(
             Rect.fromCenter(center: Offset.zero, width: s, height: s * 0.55),
             paint);