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>

src/types/storage.d.ts

@@ -10,8 +10,6 @@ export type PageType =
   | 'jwt' // JWT 解析工具
   | 'jsonDiff' // JSON 差异比较工具
   | 'base64Converter' // Base64 转换器工具
-  | 'markdownToHtml' // Markdown 转 HTML 工具
-  | 'htmlToMarkdown' // HTML 转 Markdown 工具
   | 'rightClickRestorer'; // 右键菜单恢复工具
 
 /**
@@ -29,16 +27,6 @@ export type Base64ConverterPageMode = 'text' | 'file' | 'image';
  */
 export type Base64ConvertDirection = 'encode' | 'decode';
 
-/**
- * Markdown 转 HTML 页面预览模式类型定义
- */
-export type MarkdownToHtmlPreviewMode = 'split' | 'preview' | 'html';
-
-/**
- * HTML 转 Markdown 页面预览模式类型定义
- */
-export type HtmlToMarkdownPreviewMode = 'split' | 'preview' | 'markdown';
-
 /**
  * 表单映射条目定义
  */
@@ -121,10 +109,6 @@ export interface StorageSchema {
   'base64Converter/fileMode/direction': Base64ConvertDirection;
   /** Base64 转换器「图像」子模式当前方向 */
   'base64Converter/imageMode/direction': Base64ConvertDirection;
-  /** Markdown 转 HTML 页面当前预览模式 */
-  'markdownToHtml/previewMode': MarkdownToHtmlPreviewMode;
-  /** HTML 转 Markdown 页面当前预览模式 */
-  'htmlToMarkdown/previewMode': HtmlToMarkdownPreviewMode;
   /** 语言偏好设置 */
   'app/language': string;
   /** 右键菜单待处理数据 */