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>

wxt.config.ts

@@ -30,10 +30,6 @@ function manualChunksForHtmlOnly(): Plugin {
           if (id.includes('@dnd-kit')) {
             return 'vendor-dnd';
           }
-          // Markdown 高性能转换引擎分流
-          if (id.includes('marked')) {
-            return 'vendor-markdown';
-          }
 
           return undefined;
         },