feat: remove HTML-Markdown conversion features (htmlToMarkdown & markdownToHtml)

Removed both conversion tools and all related code:
- Deleted page components: HtmlToMarkdown, MarkdownToHtml
- Deleted utility functions: htmlToMarkdown.ts, markdownToHtml.ts
- Deleted unit tests for both utilities
- Removed feature configs and PageType entries from storage.d.ts
- Removed preview mode types and StorageSchema keys
- Removed all i18n translation keys from messages.json
- Removed vendor-markdown chunk from wxt.config.ts
- Removed marked dependency from package.json
- Updated feature count in tests (11 -> 9, page order 10 -> 8)
- Updated README, AGENTS.md, and copilot-instructions.md

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

README.md

@@ -63,11 +63,6 @@
 - **文件转换**: 支持文件与 Base64 字符串互转.
 - **图像预览**: 支持图片 Base64 编码与实时预览.
 
-### 📝 Markdown ↔ HTML 转换
-
-- **Markdown 转 HTML**: 实时预览 Markdown 渲染结果，支持导出 HTML 源码.
-- **HTML 转 Markdown**: 将 HTML 内容转换为 Markdown 格式.
-
 ## 技术栈
 
 - **框架**: [WXT (Web Extension Toolkit)](https://wxt.dev/)