Replace deprecated withOpacity and surfaceVariant APIs

Migrate all withOpacity() calls to withValues(alpha:) and
surfaceVariant to surfaceContainerHighest across the codebase.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

lib/widgets/compass_indicator.dart

@@ -96,8 +96,8 @@ class _CompassIndicatorState extends State<CompassIndicator> {
               height: 52,
               decoration: BoxDecoration(
                 color: isDisabled 
-                    ? Colors.grey.withOpacity(0.8)
-                    : Colors.white.withOpacity(0.95),
+                    ? Colors.grey.withValues(alpha: 0.8)
+                    : Colors.white.withValues(alpha: 0.95),
                 shape: BoxShape.circle,
                 border: Border.all(
                   color: isDisabled 
@@ -107,7 +107,7 @@ class _CompassIndicatorState extends State<CompassIndicator> {
                 ),
                 boxShadow: [
                   BoxShadow(
-                    color: Colors.black.withOpacity(0.25),
+                    color: Colors.black.withValues(alpha: 0.25),
                     blurRadius: 6,
                     offset: const Offset(0, 3),
                   ),