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/views/admin/ProjectMembers.vue

@@ -139,13 +139,13 @@ const roleRank: Record<string, number> = {
   CRA: 40,
   IMP: 40,
 };
-const currentRoleRank = computed(() => auth.user?.role === "ADMIN" ? Number.POSITIVE_INFINITY : roleRank[projectRole.value] || 0);
-const canAssignRole = (role: string) => (auth.user?.role === "ADMIN" ? true : (roleRank[role] || 0) <= currentRoleRank.value);
+const currentRoleRank = computed(() => auth.user?.is_admin ? Number.POSITIVE_INFINITY : roleRank[projectRole.value] || 0);
+const canAssignRole = (role: string) => (auth.user?.is_admin ? true : (roleRank[role] || 0) <= currentRoleRank.value);
 const canEditMember = (row: StudyMember) => {
   if (!canManageMembers.value) return false;
-  if (auth.user?.role === "ADMIN") return true;
+  if (row.user?.is_admin) return false;
+  if (auth.user?.is_admin) return true;
   if (row.user_id === auth.user?.id) return false;
-  if (row.user?.role === "ADMIN") return false;
   return (roleRank[row.role_in_study] || 0) <= currentRoleRank.value;
 };
 
@@ -217,7 +217,7 @@ const openAdd = () => {
 const submitAdd = async () => {
   if (!projectId.value) return;
   const decision = evaluateAction({
-    actorRole: auth.user?.role || null,
+    actorRole: auth.user?.is_admin ? "ADMIN" : null,
     requiredPermission: "project.members.manage",
     target: { projectId: projectId.value },
   });
@@ -252,7 +252,7 @@ const updateRole = async (memberId: string, role: string) => {
     return;
   }
   const decision = evaluateAction({
-    actorRole: auth.user?.role || null,
+    actorRole: auth.user?.is_admin ? "ADMIN" : null,
     requiredPermission: "project.members.manage",
     target: { projectId: projectId.value, memberId },
   });
@@ -277,7 +277,7 @@ const toggleActive = async (row: StudyMember) => {
     return;
   }
   const decision = evaluateAction({
-    actorRole: auth.user?.role || null,
+    actorRole: auth.user?.is_admin ? "ADMIN" : null,
     requiredPermission: "project.members.manage",
     target: { projectId: projectId.value, memberId: row.id },
   });
@@ -316,7 +316,7 @@ const onDelete = async (row: StudyMember) => {
     return;
   }
   const decision = evaluateAction({
-    actorRole: auth.user?.role || null,
+    actorRole: auth.user?.is_admin ? "ADMIN" : null,
     requiredPermission: "project.members.manage",
     target: { projectId: projectId.value, memberId: row.id },
   });