移除用户系统级角色，权限完全迁移至项目级管理

- 用户注册/创建不再需要选择角色，账号管理只管资料和状态
- 移除前后端所有系统级 role 字段的暴露，改用 is_admin 标识管理员
- PM（项目负责人）角色自动拥有全部项目权限且不可修改
- 系统管理员在项目中的成员身份不可被删除
- 管理界面全面美化

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/users.ts

@@ -9,14 +9,13 @@ export const createUser = (payload: {
   email: string;
   password: string;
   full_name: string;
-  role: string;
   clinical_department: string;
 }) =>
   apiPost<UserInfo>("/api/v1/users/", payload);
 
 export const updateUser = (
   userId: string,
-  payload: Partial<{ role: string; status: string; password: string; full_name: string; clinical_department: string; is_active: boolean }>
+  payload: Partial<{ status: string; password: string; full_name: string; clinical_department: string; is_active: boolean }>
 ) =>
   apiPatch<UserInfo>(`/api/v1/users/${userId}`, payload);