Preserve frame position order for auto insertion

- Allow system frame position percentages to keep two decimal places without reordering saved values.

- Stop frontend and backend settings normalization from sorting framePositions on load or save.

- Capture automatic video frames in timeline order while retaining each configured position index.

- Insert automatically selected frames into report placeholders according to the configured percentage order.

- Add frame position utilities and unit coverage for two-decimal rounding, clamping, order preservation, and timeline capture planning.

- Update README, AGENTS, feature, requirement, report editor, system settings, progress, and testing docs for the new frame ordering behavior.

docs/testing.md

@@ -80,6 +80,7 @@ AI 第三方接口、讯飞语音上游 WebSocket、麦克风权限和真实视
 | 模板权限和个人模板 | 已覆盖 | `permissions.test.ts` |
 | 本地存储读写与容错 | 已覆盖 | `storage.test.ts` |
 | 系统设置混淆兼容 | 已覆盖 | `storage.test.ts` |
+| 抽帧百分比工具 | 已覆盖 | `framePositions.test.ts`，覆盖两位小数、保序和按时间生成截图计划。 |
 | 默认报告模板结构 | 已覆盖 | `defaultContent.test.ts` |
 | 默认字段和 AI Provider | 已覆盖 | `defaultContent.test.ts` |
 | AI 区域扫描工具 | 已覆盖 | `aiRegions.test.ts`，覆盖从编辑器 HTML 识别 `.ai-region` 和标题。 |