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>

package-lock.json

@@ -26,7 +26,6 @@
         "clsx": "^2.1.1",
         "dayjs": "^1.11.20",
         "lucide-react": "^1.16.0",
-        "marked": "^18.0.4",
         "qr-scanner": "^1.4.2",
         "qrious": "^4.0.2",
         "react": "^19.2.6",
@@ -8388,18 +8387,6 @@
         "url": "https://github.com/sponsors/fregante"
       }
     },
-    "node_modules/marked": {
-      "version": "18.0.4",
-      "resolved": "https://registry.npmjs.org/marked/-/marked-18.0.4.tgz",
-      "integrity": "sha512-c/BTaKzg0G6ezQx97DAkYU7k0HM6ys0FqYeKBL6hlBByZwy+ycA1+f0vDdjMHKKeEjdgkx0GOv9Il6D+85cOqA==",
-      "license": "MIT",
-      "bin": {
-        "marked": "bin/marked.js"
-      },
-      "engines": {
-        "node": ">= 20"
-      }
-    },
     "node_modules/marky": {
       "version": "1.3.0",
       "resolved": "https://registry.npmjs.org/marky/-/marky-1.3.0.tgz",