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/startup/KickoffDetail.vue

@@ -177,7 +177,7 @@ const studyId = study.currentStudy?.id || "";
 const siteInfo = reactive<{ name: string; contact?: string | null; is_active?: boolean }>({ name: "" });
 const users = ref<any[]>([]);
 const members = ref<any[]>([]);
-const isAdmin = computed(() => auth.user?.role === "ADMIN");
+const isAdmin = computed(() => auth.user?.is_admin);
 const kickoffAttachmentGroups = [
   { entityType: "startup_kickoff_minutes", title: TEXT.modules.startupMeetingAuth.kickoffMinutesLabel },
   { entityType: "startup_kickoff_signin", title: TEXT.modules.startupMeetingAuth.kickoffSignInLabel },

frontend/src/views/startup/KickoffForm.vue

@@ -78,7 +78,7 @@ const siteInfo = reactive<{ name: string; contact?: string | null; is_active?: b
 const siteId = ref("");
 const users = ref<any[]>([]);
 const members = ref<any[]>([]);
-const isAdmin = computed(() => auth.user?.role === "ADMIN");
+const isAdmin = computed(() => auth.user?.is_admin);
 
 const memberNameMap = computed(() => {
   const map: Record<string, string> = {};