feat: add About dialog to desktop app

Adds an About dialog in the top toolbar with links to the project home
page and GitHub repository. External links open via the Tauri opener
plugin so they launch in the system browser rather than the app window.

Author: giswqs

apps/geolibre-desktop/package.json

@@ -22,6 +22,7 @@
     "@tauri-apps/api": "^2.2.0",
     "@tauri-apps/plugin-dialog": "^2.2.0",
     "@tauri-apps/plugin-fs": "^2.2.0",
+    "@tauri-apps/plugin-opener": "^2.5.4",
     "mapillary-js": "^4.1.2",
     "maplibre-gl": "^5.1.1",
     "maplibre-gl-basemap-control": "^0.2.1",