fix(tui): 修复输入区粘贴丢内容、回车发送与滚轮翻页

[creatang]

恢复Markdown 渲染链路，代码块高亮与正文样式
代码块渲染异常导致的底板错位与锯齿边缘问题，输出区边缘统一。
代码块复制能力重构：
Copy 按钮调整为代码块左上角展示；
支持 fenced code 与缩进代码块两种场景自动生成复制按钮。
同步 upstream/main 并解决 TUI 冲突
修复输入区粘贴过程中内容丢失与回车误拦截问题
输入区支持滚轮翻页
修复代码块复制到输入区时内容不完整的问题
测试：go test ./...，go test ./internal/tui -cover (87.5%)
关联：Closes #119

[codecov]

Codecov Report

❌ Patch coverage is 81.56522% with 106 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/tui/update.go	76.78%	34 Missing and 18 partials ⚠️
internal/tui/copy_code.go	83.95%	18 Missing and 12 partials ⚠️
internal/tui/markdown_renderer.go	77.55%	6 Missing and 5 partials ⚠️
internal/tui/view.go	86.25%	6 Missing and 5 partials ⚠️
internal/tui/app.go	90.90%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!