refactor(core): consolidate core files under src/core directory

Move breakpoints, fonts, and app_constants into the core directory for better organization:
- Move src/lib/breakpoints.ts to src/core/breakpoints.ts
- Move src/lib/fonts.ts to src/core/fonts.ts
- Move src/core/constants/app_constants.ts to src/core/app_constants.ts and clean up outdated comments

src/core/breakpoints.ts

@@ -0,0 +1,21 @@
+/**
+ * 响应式断点常量
+ *
+ * 原始 Dart: lib/ui/core/responsive.dart
+ *
+ * 与 `src/design/breakpoints.css` 中的 CSS 变量保持一致：
+ *  - 客户端运行时检查（matchMedia）通过 `useBreakpoint` hook
+ *  - 服务端 / 客户端 CSS 通过 `src/design/breakpoints.css` 的 media query
+ */
+export const Breakpoints = {
+  /** 手机设备最大宽度 */
+  mobileMaxWidth: 500,
+  /** 平板设备最小宽度 */
+  tabletMinWidth: 600,
+  /** 平板设备最大宽度 */
+  tabletMaxWidth: 1023,
+  /** 桌面设备最小宽度 */
+  desktopMinWidth: 1024,
+} as const;
+
+export type DeviceSize = "mobile" | "tablet" | "desktop";