refactor: frame converter and mp4 track improvements

- Refactor frame converter implementation
- Update mp4 track to use ICodex
- General refactoring and code improvements

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

.cursor/rules/monibuca.mdc

@@ -0,0 +1,5 @@
+---
+description: build pb
+alwaysApply: false
+---
+如果修改了 proto 文件需要编译，请使用 scripts 目录下的脚本来编译