refactor(dev-task): 移除工时记录，改为由开始/完成日期自动计算实际工时

- 状态流转到"开发中"时自动记录 startDate
- 实际工时 = startDate → completedAt 的工作日 × 8h
- 详情抽屉移除工时记录面板，增加"开发开始日期"展示
- 列表行工时展示同步改为日期计算

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

apps/web/stores/useDevTaskStore.ts

@@ -77,8 +77,10 @@ export const useDevTaskStore = create<DevTaskState>((set, get) => ({
     if (!canTransition(task.status, to)) {
       return { ok: false, message: `不允许从「${task.status}」流转到「${to}」` };
     }
-    const completedAt = to === 'done' ? new Date().toISOString().slice(0, 10) : task.completedAt;
-    get().updateTask(id, { status: to, completedAt });
+    const today = new Date().toISOString().slice(0, 10);
+    const startDate = (to === 'in_progress' && !task.startDate) ? today : task.startDate;
+    const completedAt = to === 'done' ? today : task.completedAt;
+    get().updateTask(id, { status: to, startDate, completedAt });
     return { ok: true };
   },