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/screens/settings/sections/proximity_alerts_section.dart

@@ -140,9 +140,9 @@ class _ProximityAlertsSectionState extends State<ProximityAlertsSection> {
               Container(
                 padding: const EdgeInsets.all(12),
                 decoration: BoxDecoration(
-                  color: Colors.orange.withOpacity(0.1),
+                  color: Colors.orange.withValues(alpha: 0.1),
                   borderRadius: BorderRadius.circular(8),
-                  border: Border.all(color: Colors.orange.withOpacity(0.3)),
+                  border: Border.all(color: Colors.orange.withValues(alpha: 0.3)),
                 ),
                 child: Column(
                   crossAxisAlignment: CrossAxisAlignment.start,
@@ -233,7 +233,7 @@ class _ProximityAlertsSectionState extends State<ProximityAlertsSection> {
                     DistanceService.convertFromMeters(kProximityAlertDefaultDistance.toDouble(), appState.distanceUnit).round().toString(),
                   ]),
                   style: Theme.of(context).textTheme.bodySmall?.copyWith(
-                    color: Theme.of(context).textTheme.bodySmall?.color?.withOpacity(0.6),
+                    color: Theme.of(context).textTheme.bodySmall?.color?.withValues(alpha: 0.6),
                   ),
                 ),
               ],

lib/screens/settings/sections/tile_provider_section.dart

@@ -44,7 +44,7 @@ class TileProviderSection extends StatelessWidget {
                 ),
               )
             else
-              ...providers.map((provider) => _buildProviderTile(context, provider, appState)).toList(),
+              ...providers.map((provider) => _buildProviderTile(context, provider, appState)),
           ],
         );
       },
@@ -89,7 +89,7 @@ class TileProviderSection extends StatelessWidget {
         leading: CircleAvatar(
           backgroundColor: isSelected 
               ? Theme.of(context).colorScheme.primary
-              : Theme.of(context).colorScheme.surfaceVariant,
+              : Theme.of(context).colorScheme.surfaceContainerHighest,
           child: Icon(
             Icons.map,
             color: isSelected 

lib/screens/settings/sections/upload_mode_section.dart

@@ -81,7 +81,7 @@ class UploadModeSection extends StatelessWidget {
                               fontSize: 12, 
                               color: appState.pendingCount > 0 
                                 ? Theme.of(context).disabledColor
-                                : Theme.of(context).colorScheme.onSurface.withOpacity(0.7)
+                                : Theme.of(context).colorScheme.onSurface.withValues(alpha: 0.7)
                             )
                           );
                         case UploadMode.sandbox: