feat(portal-shell): 教师域考试管理页面迁移（P2）

按 ARCHITECTURE.md §9.1/§10 P2 要求，迁移教师域 exams 模块：
- 列表页 /shell/teacher/exams（ListPageShell + URL 状态 + 客户端二次筛选）
- 详情页 /shell/teacher/exams/[id]（DetailPageShell + 真实 exam(id) 查询）
- 新建页 /shell/teacher/exams/new（FormPageShell + MSW 兜底）
- 纯函数 transformations.ts + 19 个 vitest 单测
- @contract-pending：exams(classId) 列表查询、createExam mutation 走 MSW
- 三态 UI（loading/error/empty）+ 路由级 loading.tsx/error.tsx
- i18n：zh-CN/en 双语补全，无硬编码中文
- MSW handlers 支持 variables 透传

§11.3 DoD 验收：
- lint: 0 errors（4 个 __generated__ 预存警告）
- typecheck: 0 errors
- test: 250/250 passed（含 19 个新增 transformations 测试）
- lint:tokens: 0 errors

apps/portal-shell/src/messages/en.json

@@ -146,7 +146,71 @@
     "title": "Classes"
   },
   "exams": {
-    "title": "Exams"
+    "title": "Exams",
+    "list": {
+      "title": "Exams",
+      "description": "View and manage all exams",
+      "new": "New Exam",
+      "searchPlaceholder": "Search exam name...",
+      "statusFilter": "Filter by status",
+      "statusAll": "All statuses",
+      "statusDraft": "Draft",
+      "statusPublished": "Published",
+      "statusInProgress": "In Progress",
+      "statusScored": "Completed",
+      "total": "{count} total",
+      "colName": "Name",
+      "colStatus": "Status",
+      "colExamDate": "Exam Date",
+      "colDuration": "Duration",
+      "colTotalScore": "Total Score",
+      "colActions": "Actions",
+      "viewDetail": "View Detail →",
+      "mswNotice": "List query contract is pending, please ensure NEXT_PUBLIC_MSW=1 is enabled.",
+      "unitScore": "pts"
+    },
+    "detail": {
+      "title": "Exam Detail",
+      "edit": "Edit",
+      "notFound": "Exam not found, may have been deleted",
+      "createdAtPrefix": "Created on {date}",
+      "sectionBasic": "Basic Info",
+      "sectionStatus": "Status Info",
+      "fieldTitle": "Title",
+      "fieldDescription": "Description",
+      "fieldExamDate": "Exam Date",
+      "fieldDuration": "Duration",
+      "fieldTotalScore": "Total Score",
+      "fieldClassId": "Class ID",
+      "fieldSubjectId": "Subject ID",
+      "fieldCurrentStatus": "Current Status",
+      "fieldStatusChangedAt": "Status Changed At",
+      "fieldStatusChangedBy": "Status Changed By",
+      "fieldCreatedBy": "Created By",
+      "fieldCreatedAt": "Created At",
+      "fieldUpdatedAt": "Updated At",
+      "unitScore": "pts"
+    },
+    "new": {
+      "title": "New Exam",
+      "description": "Fill in exam basic information",
+      "submit": "Create Exam",
+      "success": "Exam created successfully",
+      "error": "Creation failed",
+      "classId": "Class ID",
+      "subjectId": "Subject ID",
+      "titleLabel": "Title",
+      "titlePlaceholder": "e.g. 2026 Spring Midterm",
+      "descriptionLabel": "Description",
+      "descriptionPlaceholder": "Exam scope, notes, etc.",
+      "examDate": "Exam Date",
+      "duration": "Duration (minutes)",
+      "totalScore": "Total Score",
+      "errorClassRequired": "Please fill in Class ID",
+      "errorTitleRequired": "Please fill in exam title",
+      "errorDateRequired": "Please select exam date",
+      "contractPending": "Create exam contract is @contract-pending, currently backed by MSW. Will switch to real submission once backend mutation is ready."
+    }
   },
   "homework": {
     "title": "Homework"