feat(nav): 新增前台公开导航页与后台导航管理

新增模块
- Nav 模型（applications/models/admin_nav.py）：公开导航表，与 admin_power 解耦
- NavOutSchema / NavManageSchema（applications/schemas/admin_nav.py）
- 前台公开视图 applications/view/public/nav.py：/site/、/site/category/<name>、/site/api/navs，无需登录
- 后台管理视图 applications/view/system/nav.py：CRUD + 启用/禁用，挂入 system_bp
- 前台模板 templates/public/{base,index,category}.html（独立配色，无需登录即可访问）
- 后台模板 templates/system/nav/{main,add,edit}.html（Layui table + form）

调整
- applications/view/__init__.py：注册前台蓝图到根
- applications/view/system/__init__.py：注册后台 nav 子蓝图
- applications/common/script/admin.py：admin init/export 支持 navs 数组
- applications/common/script/app_data/init.json：新增 10 条导航种子 + 4 条 system:nav 权限

数据库
- 新增 public_nav 表（含 category 索引）
- migration: 38751d88d42c_add_public_nav_table.py

applications/view/system/__init__.py

@@ -6,6 +6,7 @@ from applications.view.system.index import bp as index_bp
 from applications.view.system.log import bp as log_bp
 from applications.view.system.mail import bp as mail_bp
 from applications.view.system.monitor import bp as monitor_bp
+from applications.view.system.nav import bp as nav_bp
 from applications.view.system.passport import bp as passport_bp
 from applications.view.system.power import bp as power_bp
 from applications.view.system.rights import bp as right_bp
@@ -30,5 +31,6 @@ def register_system_bps(app: Flask):
     system_bp.register_blueprint(passport_bp)
     system_bp.register_blueprint(right_bp)
     system_bp.register_blueprint(dept_bp)
+    system_bp.register_blueprint(nav_bp)
     app.register_blueprint(index_bp)
     app.register_blueprint(system_bp)