feat(exams,homework,proctoring): 长期问题修复与竞品差距补齐

P1-1 跨模块直查消除：
- homework/data-access-classes.ts 移除对 exams/subjects 表的 JOIN 直查
- 改为调用 exams/data-access.getExamSubjectIdMap + school/data-access.getSubjectNameMapByIds
- school/data-access.ts 新增 getSubjectNameMapByIds 批量科目名称映射函数

P1-2 as 断言消除（exam-mode-config.tsx）：
- 移除全部 10 处 as 类型断言
- 改用 useFormContext 替代 Control prop，避免 Control<T> 不变型问题
- exam-form.tsx 调用方简化为 <ExamModeConfig />（已集成到考试表单）

P1-3 as 断言消除（proctoring-dashboard.tsx）：
- 用类型守卫函数 isProctoringEventType + toProctoringEventTypes
  替代 Object.keys(...) as ProctoringEventType[] 断言

P0-竞品倒计时（对标智学网/猿题库）：
- 新增 hooks/use-exam-countdown.ts 考试倒计时 Hook
- homework-take-view.tsx 集成限时/监考模式倒计时显示与到时自动提交
- data-access.ts 的 getStudentHomeworkTakeData 新增 examModeConfig + startedAt 字段
- types.ts 扩展 StudentHomeworkTakeData 类型
- i18n 补充 timedExam/timeRemaining/timeUpAutoSubmit 翻译键

架构文档同步：
- 004/005 更新 homework/proctoring/school/exams 模块导出与依赖关系
- 005 新增 homework.hooks.useExamCountdown 与 school.dataAccess.getSubjectNameMapByIds
- 005 依赖矩阵 homework→school 补充 getSubjectNameMapByIds

验证：tsc --noEmit 零错误，eslint 零错误（3 个预存 warning 无关）

src/shared/i18n/messages/en/exam-homework.json

@@ -197,6 +197,9 @@
       "readyDescription": "Click the \"Start Assignment\" button above to begin. Your answers will be saved when you click \"Save Answer\".",
       "startNow": "Start Now",
       "back": "Back",
+      "timedExam": "Timed exam: {{minutes}} minutes",
+      "timeRemaining": "Time remaining",
+      "timeUpAutoSubmit": "Time is up, auto-submitting...",
       "confirmSubmit": "Confirm Submission",
       "confirmSubmitDescription": "All questions have been answered. Submitted answers cannot be changed. Are you sure you want to submit?",
       "unansweredWarning": "You have {{count}} unanswered question(s). Submitted answers cannot be changed. Are you sure you want to submit?",