feat: 进行功能分区方便后期维护

- 添加导航条目和分类数据模型
- 实现导航卡片网格展示
- 支持点击卡片打开链接
- 添加主题样式（渐变色、卡片阴影）
- 区分系统默认和自定义条目
- 添加分类统计信息"

frontend/test/core/constants/app_breakpoints_test.dart

@@ -0,0 +1,32 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:frontend/core/constants/app_breakpoints.dart';
+
+void main() {
+  group('AppBreakpoints.gridColumnsFor', () {
+    test('宽度不足一个单元时退化为单列', () {
+      expect(AppBreakpoints.gridColumnsFor(200), 1);
+      expect(AppBreakpoints.gridColumnsFor(220), 1);
+    });
+
+    test('刚好放下两列与放不下时的边界', () {
+      // 两列 = 220 * 2 + 16 = 456
+      expect(AppBreakpoints.gridColumnsFor(456), 2);
+      expect(AppBreakpoints.gridColumnsFor(455), 1);
+    });
+
+    test('宽度越大列数越多', () {
+      expect(AppBreakpoints.gridColumnsFor(900), 3);
+      expect(AppBreakpoints.gridColumnsFor(1200), 5);
+    });
+
+    test('与 CSS 的 auto-fill 规则一致', () {
+      // 列数 = floor((可用宽度 + 间距) / (最小单元 + 间距))
+      for (final double width in <double>[300, 520, 740, 1080, 1440]) {
+        final int columns = AppBreakpoints.gridColumnsFor(width);
+        final double used = columns * 220 + (columns - 1) * 16;
+        expect(used, lessThanOrEqualTo(width));
+        expect(used + 236, greaterThan(width));
+      }
+    });
+  });
+}