perf: slim per-card build cost

Memoize the per-image SHA-1 disk-cache key (ran on the UI thread on
every image build), fade images in via paint alpha instead of an
AnimatedSwitcher saveLayer per in-flight tile, create SettingsBuilder's
merged listenable once instead of per build, and drop the pointer
chrome (InkWell ink machinery, cursor MouseRegions) from cards on TV
where no pointer exists.

lib/widgets/clickable_cursor.dart

@@ -1,5 +1,7 @@
 import 'package:flutter/material.dart';
 
+import '../utils/platform_detector.dart';
+
 class ClickableCursor extends StatelessWidget {
   final Widget child;
   final bool enabled;
@@ -8,6 +10,9 @@ class ClickableCursor extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
+    // No pointer on TV: skip the MouseRegion entirely — one exists per card
+    // and they add up on low-end devices.
+    if (PlatformDetector.isTV()) return child;
     return MouseRegion(cursor: enabled ? SystemMouseCursors.click : MouseCursor.defer, child: child);
   }
 }