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- Move src/core/constants/app_constants.ts to src/core/app_constants.ts and clean up outdated comments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/app_constants.ts

@@ -0,0 +1,37 @@
+import { getAppEnv } from "@/core/net/config/api_config";
+
+export class AppConstants {
+  private constructor() {}
+
+  /** 隐私政策文档链接 */
+  static readonly privacyPolicyUrl =
+    "https://docs.google.com/document/d/1KyNkbnZ5-2gNUFlE0Jd9eqp-MGKOc95OBAa6hrxK1wg/edit?usp=sharing";
+
+  /** 服务条款文档链接 */
+  static readonly termsOfServiceUrl =
+    "https://docs.google.com/document/d/1ZL5-uDKLNQrkNnWNKlVFi5atwA9hBpNVyoXzzXf1GHs/edit?usp=sharing";
+
+  /** 开发环境 APP 标题 */
+  static readonly appTitleDevelopment = "Develop";
+
+  /** 测试环境 APP 标题 */
+  static readonly appTitleTest = "Test";
+
+  /** 生产环境 APP 标题 */
+  static readonly appTitleProduction = "Cozsweet";
+
+  /** 根据当前环境获取 APP 标题 */
+  static get appTitle(): string {
+    const env = getAppEnv();
+    if (env === "production") return AppConstants.appTitleProduction;
+    if (env === "test") return AppConstants.appTitleTest;
+    return AppConstants.appTitleDevelopment;
+  }
+
+  /** APP 站点 URL（按环境区分：生产用主站域名，其余用前端测试域名） */
+  static get appUrl(): string {
+    return getAppEnv() === "production"
+      ? "https://cozsweet.banlv-ai.com"
+      : "https://frontend-test.banlv-ai.com";
+  }
+}