refactor: simplify export to PNG-only, improve controls UI

- Replace html2canvas with modern-screenshot for better performance
- Remove ExportModal and CSS export functionality
- Simplify export to single PNG download
- Improve controls layout with better spacing and text shadows
- Increase glass-panel opacity for better readability
- Remove unused ExportData interface and CSS generation utilities

package.json

@@ -23,8 +23,8 @@
     "@tailwindcss/vite": "^4.1.17",
     "cssnano": "^7.1.0",
     "framer-motion": "^12.23.24",
-    "html2canvas": "^1.4.1",
     "lucide-preact": "^0.468.0",
+    "modern-screenshot": "^4.6.6",
     "preact": "^10.27.2",
     "preact-iso": "^2.11.0",
     "preact-render-to-string": "^6.6.3",