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/requirements.md

@@ -60,7 +60,7 @@
 
 ### 系统设置
 
-- 超级管理员可配置抽帧数量、抽帧百分比、抽帧计算模式和自动插入策略。
+- 超级管理员可配置抽帧数量、两位小数抽帧百分比、抽帧计算模式和自动插入策略；百分比配置顺序用于自动插入顺序。
 - 超级管理员可配置 AI 服务商、接口地址、API Key 和模型名。
 - 超级管理员可配置讯飞语音听写参数。
 - 所有角色可设置默认报告模板。