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/lib/dev-task.ts

@@ -85,6 +85,22 @@ export function formatHours(hours: number): string {
   return `${hours}h（≈${days}人天）`;
 }
 
+export function calcActualHoursByDates(startDate?: string, completedAt?: string): number {
+  if (!startDate) return 0;
+  const end = completedAt || new Date().toISOString().slice(0, 10);
+  const start = new Date(startDate);
+  const endDate = new Date(end);
+  if (isNaN(start.getTime()) || isNaN(endDate.getTime()) || endDate < start) return 0;
+  let workDays = 0;
+  const current = new Date(start);
+  while (current <= endDate) {
+    const day = current.getDay();
+    if (day !== 0 && day !== 6) workDays++;
+    current.setDate(current.getDate() + 1);
+  }
+  return workDays * 8;
+}
+
 export function generateTaskNo(existingTasks: DevTask[]): string {
   const maxNum = existingTasks.reduce((max, t) => {
     const num = parseInt(t.taskNo.replace('DEV-', ''), 10);