test(client): 用例名统一为 should_<期望>_when_<条件>（155/155）

为什么：红线第 8 条门禁立即生效，要求 Flutter 测试命名合规；此前 155 条用例名是英文自然语言
或中文描述，合规数 0/155，属存量欠账，按"先补齐再上新功能"在本批处理。

- 25 个测试文件、155 条用例改名，按「文件 + 旧名」精确定位，每条断言命中恰好 1 次才替换
- 只改名字字面量：改动行 155 删 + 155 增，无一行不含字符串字面量（断言与用例体零改动）
- 门禁：dart analyze lib test = No issues found；flutter test = 155 全过

test/playback/seek_progress_bar_test.dart

@@ -4,7 +4,7 @@ import 'package:nontonki/pages/player/widgets/seek_progress_bar.dart';
 import 'package:video_player/video_player.dart';
 
 void main() {
-  test('seek target stays inside playable duration', () {
+  test('should_keep_seek_target_inside_playable_duration_when_dragging', () {
     const duration = Duration(seconds: 12);
     expect(seekPositionForFraction(0, duration), Duration.zero);
     expect(
@@ -14,7 +14,7 @@ void main() {
     expect(seekPositionForFraction(1, duration), lessThan(duration));
   });
 
-  testWidgets('horizontal slider drag previews and seeks on release', (
+  testWidgets('should_preview_while_dragging_and_seek_on_release_when_slider_is_horizontal', (
     tester,
   ) async {
     final video = ValueNotifier<VideoPlayerValue>(
@@ -61,7 +61,7 @@ void main() {
     expect(target!, lessThan(const Duration(seconds: 12)));
   });
 
-  testWidgets('compact seek bar shows time only while dragging', (
+  testWidgets('should_show_time_only_while_dragging_when_seek_bar_is_compact', (
     tester,
   ) async {
     final video = ValueNotifier<VideoPlayerValue>(