feat(dashboard): optimize teacher dashboard ui and layout

- Refactor layout: move Needs Grading to main column, Homework to sidebar
- Enhance TeacherStats: replace static counts with actionable metrics (Needs Grading, Active Assignments, Avg Score, Submission Rate)
- Update RecentSubmissions: table view with quick grade actions and late status
- Update TeacherSchedule: vertical timeline view with scroll hints
- Update TeacherHomeworkCard: compact list view
- Integrate Recharts: add TeacherGradeTrends chart and shared chart component
- Update documentation

docs/architecture/003_frontend_engineering_standards.md

@@ -419,6 +419,7 @@ export type ActionState<T = void> = {
   - 禁止在 CSS 中 `@import` 外部字体 URL（避免 CLS 与阻塞渲染）
 - 依赖：
   - 禁止引入重型动画库作为默认方案；复杂动效需按需加载并解释收益
+  - **图表**：标准图表库统一使用 `recharts`（通过 `src/shared/components/ui/chart.tsx` 封装），禁止引入其他图表库（如 Chart.js / Highcharts）。
   - 大体积 Client 组件必须拆分与动态加载，并通过 `Suspense` 提供 skeleton fallback
 
 ---