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/components/FaqCategoryPanel.vue

@@ -51,7 +51,7 @@ const showForm = ref(false);
 const editing = ref<FaqCategory | null>(null);
 
 const { can } = usePermission();
-const isAdmin = computed(() => auth.user?.role === "ADMIN");
+const isAdmin = computed(() => !!auth.user?.is_admin);
 const canMaintain = computed(() => can("faq.edit"));
 const canDelete = computed(() => isAdmin.value);
 

frontend/src/components/Layout.vue

@@ -232,7 +232,7 @@ const auth = useAuthStore();
 const study = useStudyStore();
 const router = useRouter();
 const route = useRoute();
-const isAdmin = computed(() => auth.user?.role === "ADMIN");
+const isAdmin = computed(() => !!auth.user?.is_admin);
 const isCollapsed = ref(localStorage.getItem("ctms_sidebar_collapsed") === "1");
 const projectRole = computed(() => study.currentStudyRole || (study.currentStudy as any)?.role_in_study || "");
 const canAccessProjectPath = (path: string) =>