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/progress.md

@@ -90,3 +90,4 @@
 | 2026-05-02 | 移除前端用户可见 JSON 导出入口，保留模板历史 JSON 导入兼容。 |
 | 2026-05-02 | 模板 HTML 导出包补充模板字段和字段管理设置，导入时恢复字段库元数据。 |
 | 2026-05-02 | 修复报告编辑器加载已有 AI 区域后下拉栏初始显示“无可用 AI 区域”的问题。 |
+| 2026-05-02 | 调整抽帧百分比为两位小数保序保存；自动截图按时间顺序执行，自动插入按配置顺序执行。 |