feat: refresh UI with new typography and edit diff display (#63)

- Switch from Geist to Plus Jakarta Sans (body) and JetBrains Mono (code)
- Add visual diff display for edit_diagram tool showing search/replace pairs
- Update color palette to clean modern OKLCH-based scheme
- Improve chat message display with better styling and animations
- Add syntax-highlighted code blocks for XML/JSON output
- Improve scrollbar and shadow utilities

package.json

@@ -33,6 +33,7 @@
         "next": "15.2.3",
         "ollama-ai-provider-v2": "^1.5.4",
         "pako": "^2.1.0",
+        "prism-react-renderer": "^2.4.1",
         "react": "^19.0.0",
         "react-dom": "^19.0.0",
         "react-drawio": "^1.0.3",