feat: add PNG export, text alignment, and font size controls

Massive UX improvements for preview customization and export:

**PNG Export**
- Export ASCII art as high-quality PNG images
- Uses html-to-image library (2x pixel ratio)
- Preserves background color and styling
- Auto-download with font name in filename
- Toast notification on success/error
- PNG button in preview toolbar

**Text Alignment Controls**
- Left / Center / Right alignment buttons
- Visual toggle buttons with icons
- Live preview updates
- Persists during font/text changes
- Smooth transitions

**Font Size Controls**
- XS (10px) / SM (12-14px) / MD (14-16px)
- Toggle buttons for quick switching
- Responsive font sizes
- Better readability options
- Great for different screen sizes

**Enhanced Preview Toolbar**
- Reorganized button layout
- Better button labels (Copy, Share, PNG, TXT)
- Tooltips on all buttons
- Icon + text labels
- Wrapped flex layout for mobile

**Preview Controls UI**
- Two control groups (align + size)
- Bordered button groups
- Active state highlighting
- Hover states
- Clean, compact design

**Updated Keyboard Shortcuts Help**
- Better descriptions
- Added "Tips" section
- Feature discovery hints
- Grouped by category
- More helpful content

**Technical Improvements**
- Added html-to-image dependency
- Ref-based element capture
- Dynamic className composition
- State management for controls
- Proper TypeScript types

**Button Improvements**
- "Download" → "TXT" (more specific)
- Added "PNG" button
- Better icon usage
- Consistent sizing
- Mobile-friendly layout

The preview is now fully customizable with professional export options!

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

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

package.json

@@ -12,6 +12,7 @@
     "clsx": "^2.1.1",
     "figlet": "^1.8.0",
     "fuse.js": "^7.1.0",
+    "html-to-image": "^1.11.13",
     "lucide-react": "^0.553.0",
     "next": "^16.0.0",
     "react": "^19.0.0",