feat: Implement Google Docs-style editor with ODT export

This commit introduces a complete overhaul of the application, replacing the
previous simple Markdown editor with a powerful, Google Docs-style
editor with a live preview and ODT export.

The new features include:
- A new three-pane UI with a Markdown editor, a live preview, and a
  styles panel.
- A powerful styling engine that allows users to create, edit, and
  apply custom styles to the document.
- A persistent storage mechanism for styles using a JSON file.
- Support for advanced Markdown features, including code blocks and
  blockquotes.
- A hyperlinked table of contents that is automatically generated from
  the headings in the document.
- Auto-captioning for tables and images.
- Pagination for large tables.
- Secure image embedding with a domain whitelist and size limits to
  prevent SSRF and DoS attacks.
- An improved live preview that correctly renders multi-line Markdown
  elements.

requirements.txt

@@ -1,3 +1,5 @@
 Flask==2.3.3
-python-docx==0.8.11
-markdown==3.4.4
+gunicorn==23.0.0
+markdown==3.4.4
+odfdo==3.17.6
+requests