feat: Make app fully portable

- Add tauri-plugin-store for portable data storage
- Implement portable data directory (TypoGenie-Data/ next to EXE)
- Configure Rust backend to use EXE-relative paths
- Add store permissions for persistent settings
- Update README with portable badges and documentation
- Document how to build and use the portable EXE
- Zero registry, zero AppData, fully self-contained

package.json

@@ -17,6 +17,7 @@
     "@tauri-apps/api": "^2.0.0",
     "@tauri-apps/plugin-dialog": "^2.0.0",
     "@tauri-apps/plugin-fs": "^2.0.0",
+    "@tauri-apps/plugin-store": "^2.4.2",
     "docx": "^8.5.0",
     "lucide-react": "^0.563.0",
     "marked": "12.0.0",