fix: 实际耗时改为精确时间戳计算（精确到0.5h）

之前用日期+工作日×8h 计算耗时太粗糙，现改为：
- DevTask startDate/completedAt 使用完整 ISO 时间戳（含时分秒）
- TestCase startedAt/completedAt 同上
- 耗时 = (endTimestamp - startTimestamp) / 3600000，精确到0.5h
- 详情抽屉显示精确到分钟的时间（YYYY-MM-DD HH:mm）

例如：08:00 开始，10:30 提测，耗时 2.5h（不再是8h）

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

apps/web/lib/dev-task.ts

@@ -83,18 +83,12 @@ export function formatHours(hours: number): string {
 
 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;
+  const end = completedAt || new Date().toISOString();
+  const startMs = new Date(startDate).getTime();
+  const endMs = new Date(end).getTime();
+  if (isNaN(startMs) || isNaN(endMs) || endMs < startMs) return 0;
+  const diffHours = (endMs - startMs) / (1000 * 60 * 60);
+  return Math.round(diffHours * 2) / 2; // 精确到0.5h
 }
 
 export function generateTaskNo(existingTasks: DevTask[]): string {