feat: implement insert mode with heading protection and fix content display bugs

This commit implements a comprehensive insert mode that preserves document structure
by protecting heading levels 1-3 from modification while allowing full content editing.

## Insert Mode Features
- CLI integration with --insert flag for md-render command
- Protected heading display (read-only) for levels 1-3
- Content-only editing for sections with protected headings
- Full editing capability for heading levels 4-6
- Theme-aware CSS styling for all UI themes
- Modal confirmation dialogs with proper positioning
- Section splitting with automatic protection inheritance
- Validation to prevent protected heading modifications

## Implementation Details
- Added MARKITECT_INSERT_MODE JavaScript flag and configuration
- Enhanced Section class with heading level detection and protection methods
- Added getHeadingText() and getHeadingContent() methods for content separation
- Implemented insert mode UI with protected heading display above content editor
- Added comprehensive CSS styling for insert mode components and modals
- Updated CLI with --insert option and mutual exclusion with --edit

## Bug Fixes
- Fixed JavaScript syntax errors caused by unescaped newline characters in string literals
- Corrected split('\n') and join('\n') calls to use proper escaping for Python string context
- Fixed heading level 3 display showing "null" by improving regex pattern matching
- Resolved content not displaying in edit/insert modes due to JavaScript parsing failures

## Documentation
- Updated UserInterfaceFramework.md with complete Insert Mode Editor section
- Added behavioral comparison table between edit and insert modes
- Updated Component Integration Matrix to reflect new capabilities

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

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

markitect/plugins/builtin/markdown_commands.py

@@ -1961,6 +1961,8 @@ def md_list_command(ctx, output_format, names_only):
               help='Custom CSS file to include')
 @click.option('--edit', is_flag=True,
               help='Open in interactive edit mode with stable section editing')
+@click.option('--insert', is_flag=True,
+              help='Open in interactive insert mode with heading protection (levels 1-3 read-only)')
 @click.option('--editor-theme', default='github',
               type=click.Choice(['github', 'monokai', 'tomorrow', 'dark']),
               help='Editor theme for live edit mode (default: github)')
@@ -1973,7 +1975,7 @@ def md_list_command(ctx, output_format, names_only):
 @click.option('--nodogtag', is_flag=True,
               help='Don\'t add HTML generation dogtag at end of document')
 @click.pass_context
-def md_render_command(ctx, input_file, output, theme, css, edit, editor_theme,
+def md_render_command(ctx, input_file, output, theme, css, edit, insert, editor_theme,
                      keyboard_shortcuts, use_publication_dir, dont_use_publication_dir, nodogtag):
     """
     Render a markdown file to HTML with basic templates and live preview capabilities.
@@ -1992,6 +1994,7 @@ def md_render_command(ctx, input_file, output, theme, css, edit, editor_theme,
         markitect md-render README.md
         markitect md-render docs/guide.md --output guide.html --theme github
         markitect md-render draft.md --edit --editor-theme monokai
+        markitect md-render draft.md --insert --editor-theme monokai
         markitect md-render doc.md --theme dark --css custom.css
         markitect md-render doc.md --theme dark,academic
         markitect md-render doc.md --theme light,github,corporate
@@ -2001,6 +2004,10 @@ def md_render_command(ctx, input_file, output, theme, css, edit, editor_theme,
     try:
         input_path = Path(input_file)
 
+        # Validate mode flags
+        if edit and insert:
+            raise click.BadParameter("Cannot use both --edit and --insert flags simultaneously. Choose one mode.")
+
         # Determine output path
         if output:
             output_path = Path(output)
@@ -2034,10 +2041,29 @@ def md_render_command(ctx, input_file, output, theme, css, edit, editor_theme,
                 click.echo(f"Keyboard shortcuts: {'enabled' if keyboard_shortcuts else 'disabled'}")
                 click.echo(f"Theme: {theme or 'default'}")
                 click.echo(f"CSS: {css or 'default'}")
+        elif insert:
+            # Insert mode - generate HTML with insert capabilities and heading protection
+            result = doc_manager.render_file(input_file, str(output_path),
+                                           template=theme, css=css,
+                                           insert_mode=True,
+                                           editor_theme=editor_theme,
+                                           keyboard_shortcuts=keyboard_shortcuts,
+                                           nodogtag=nodogtag)
+
+            click.echo(f"✓ Rendered with interactive insert capabilities to: {output_path}")
+
+            if config.get('verbose', False):
+                click.echo(f"Editor theme: {editor_theme}")
+                click.echo(f"Keyboard shortcuts: {'enabled' if keyboard_shortcuts else 'disabled'}")
+                click.echo(f"Heading protection: levels 1-3 read-only")
+                click.echo(f"Theme: {theme or 'default'}")
+                click.echo(f"CSS: {css or 'default'}")
         else:
             # Static render
             result = doc_manager.render_file(input_file, str(output_path),
                                            template=theme, css=css,
+                                           edit_mode=False,
+                                           insert_mode=False,
                                            nodogtag=nodogtag)
             click.echo(f"✓ Rendered to: {output_path}")