fix: resolve CSS embedding import error in HTML template generation

Fixed critical bug where CSS files were not being embedded inline due to
missing Path import in _generate_html_template method. This was causing
CSS content to fall back to link tags instead of inline embedding.

The issue was:
- Path used in CSS handling code (line 367) before being imported
- Path import was conditional and occurred later in the method (line 623)
- Exception handling silently fell back to link tags

Solution:
- Added explicit `from pathlib import Path` import at method start
- CSS files now properly embed inline as intended
- All CSS-related tests now pass (6/6 previously failing)

This resolves the test failures where CSS content was expected to be
embedded inline but was generating link tags instead.

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

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

markitect/document_manager.py

@@ -355,6 +355,7 @@ class DocumentManager:
                               edit_mode: bool = False, editor_theme: str = 'github', keyboard_shortcuts: bool = True) -> str:
         """Generate HTML template with embedded markdown and client-side rendering."""
         import json
+        from pathlib import Path
 
         # Escape the markdown content for JavaScript
         js_markdown_content = json.dumps(markdown_content)