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>

AGENTS.md

@@ -142,7 +142,6 @@ src/pages/FeatureName/
 
 - `@webext-core/messaging` — 扩展消息通信
 - `@dnd-kit` — 拖拽排序（用于页面顺序管理）
-- `marked` — Markdown 解析
 - `qrious` + `qr-scanner` — 二维码生成与解析
 - `dayjs` — 日期处理（时间戳转换）
 - `sonner` — Toast 通知（替代传统 snackbar）