feat: adopt material 3 theme and billing adjustments

lib/widgets/category_header_widget.dart

@@ -36,27 +36,27 @@ class CategoryHeaderWidget extends StatelessWidget {
             children: [
               Text(
                 categoryName,
-                style: const TextStyle(
+                style: TextStyle(
                   fontSize: 16,
                   fontWeight: FontWeight.w700,
-                  color: Color(0xFF374151),
+                  color: Theme.of(context).colorScheme.onSurface,
                 ),
               ),
               Text(
                 _buildCostDisplay(context),
-                style: const TextStyle(
+                style: TextStyle(
                   fontSize: 12,
                   fontWeight: FontWeight.w500,
-                  color: Color(0xFF6B7280),
+                  color: Theme.of(context).colorScheme.onSurfaceVariant,
                 ),
               ),
             ],
           ),
           const SizedBox(height: 8),
-          const Divider(
+          Divider(
             height: 1,
             thickness: 1,
-            color: Color(0xFFEEEEEE),
+            color: Theme.of(context).colorScheme.outline.withValues(alpha: 0.3),
           ),
         ],
       ),