feat: add comprehensive DOCX document support

- Install docx (v9.5.1) and mammoth (v1.11.0) packages
- Create docxService.ts with full DOCX read/write functionality:
  - Extract text, HTML, and Markdown from DOCX files using mammoth
  - Generate DOCX files from Markdown with proper heading levels (H1-H3)
  - Generate DOCX files from HTML and plain text
  - Automatic paragraph formatting and spacing
- Integrate DOCX conversions into pandocService.ts
- Update README with DOCX support documentation
- Add DOCX libraries to tech stack section

Supported DOCX conversions:
- DOCX → Text/HTML/Markdown
- Markdown/HTML/Text → DOCX

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -13,10 +13,12 @@
     "@ffmpeg/util": "^0.12.1",
     "@imagemagick/magick-wasm": "^0.0.30",
     "clsx": "^2.1.1",
+    "docx": "^9.0.4",
     "dompurify": "^3.2.2",
     "fuse.js": "^7.1.0",
     "jspdf": "^2.5.2",
     "lucide-react": "^0.553.0",
+    "mammoth": "^1.8.0",
     "marked": "^15.0.4",
     "next": "^16.0.0",
     "pdfjs-dist": "^4.10.38",