feat: enhance CMS editor with rich text features and fix line break preservation

- Fix line break preservation with proper HardBreak extension configuration
- Add text alignment support (left, center, right, justify) for paragraphs and headings
- Add H4-H6 heading support with keyboard shortcuts
- Add code block support with syntax highlighting for multiple languages
- Enhance preview functionality with split-pane view (Edit/Preview/Split modes)
- Add custom prose CSS overrides for proper content rendering
- Update DOMPurify configuration to preserve br tags and text alignment
- Add visual indicators and help modal with all keyboard shortcuts
- Improve toolbar organization with grouped buttons and separators
- Add auto-save status indicator and word/character count

Fixes issues with:
- Line breaks not showing in gallery view
- Limited heading options (only H1-H3)
- No text formatting capabilities
- Missing preview functionality

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

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

frontend/src/main.tsx

@@ -1,6 +1,7 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
 import './index.css'
+import './styles/prose-overrides.css'
 import './i18n/config'
 import App from './App.tsx'