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/utils/roles.test.ts

@@ -1,10 +1,11 @@
 import { describe, expect, it } from "vitest";
-import { getProjectRole, getSystemRole, isSystemAdmin } from "./roles";
+import { getProjectRole, isSystemAdmin } from "./roles";
 
 describe("role helpers", () => {
   it("keeps system admin separate from project admin", () => {
-    expect(isSystemAdmin({ role: "ADMIN" } as any)).toBe(true);
-    expect(getSystemRole({ role: "ADMIN" } as any)).toBe("ADMIN");
+    expect(isSystemAdmin({ is_admin: true } as any)).toBe(true);
+    expect(isSystemAdmin({ is_admin: false } as any)).toBe(false);
+    expect(isSystemAdmin(null)).toBe(false);
     expect(getProjectRole({ role_in_study: null } as any, null)).toBeNull();
   });
 

frontend/src/utils/roles.ts

@@ -1,8 +1,6 @@
 import type { Study, UserInfo } from "../types/api";
 
-export const getSystemRole = (user?: Pick<UserInfo, "role"> | null) => user?.role || null;
-
-export const isSystemAdmin = (user?: Pick<UserInfo, "role"> | null) => getSystemRole(user) === "ADMIN";
+export const isSystemAdmin = (user?: Pick<UserInfo, "is_admin"> | null) => !!user?.is_admin;
 
 export const getProjectRole = (study?: Pick<Study, "role_in_study"> | null, currentStudyRole?: string | null) =>
   currentStudyRole || study?.role_in_study || null;