fix: rewrite document converter to preserve formatting in PDF output

Replace jsPDF.text() (plain text dump) with jsPDF.html() + html2canvas-pro
which renders actual styled DOM elements into the PDF. DOCX headings, bold,
italic, tables, lists, links now render correctly. All HTML output also gets
a full styled document wrapper with embedded CSS. HTML-to-Markdown converter
rewritten with proper DOM-walking parser instead of fragile regex.

package.json

@@ -13,6 +13,7 @@
     "@ffmpeg/util": "^0.12.2",
     "fast-xml-parser": "^5.4.2",
     "framer-motion": "^12.35.2",
+    "html2canvas-pro": "^2.0.2",
     "js-yaml": "^4.1.1",
     "jspdf": "^4.2.0",
     "jszip": "^3.10.1",