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/stores/useTestCaseStore.ts

@@ -74,14 +74,12 @@ export const useTestCaseStore = create<TestCaseState>((set, get) => ({
     if (!canTcTransition(tc.status, to)) {
       return { ok: false, message: `不允许从「${tc.status}」流转到「${to}」` };
     }
-    const today = new Date().toISOString().slice(0, 10);
+    const now = new Date().toISOString();
     const patch: Partial<TestCase> = { status: to };
-    // 开始执行时间
-    if (to === 'running' && !tc.startedAt) patch.startedAt = today;
-    // 执行完成时间（通过/失败/阻塞都算执行完成）
-    if (to === 'passed' || to === 'failed' || to === 'blocked') patch.completedAt = today;
+    if (to === 'running' && !tc.startedAt) patch.startedAt = now;
+    if (to === 'passed' || to === 'failed' || to === 'blocked') patch.completedAt = now;
     if (to === 'running' || to === 'passed' || to === 'failed' || to === 'blocked') {
-      patch.executedAt = today;
+      patch.executedAt = now;
     }
     // 回退到执行中时清除完成时间
     if (to === 'running') { patch.failReason = undefined; patch.blockReason = undefined; patch.completedAt = undefined; }