fix: replace removed Github icon with simple-icons

lucide-svelte 1.0 dropped brand icons, breaking the Github import in
AboutModal. Use @icons-pack/svelte-simple-icons (SiGithub) instead.

package.json

@@ -26,6 +26,7 @@
   },
   "license": "MIT",
   "dependencies": {
+    "@icons-pack/svelte-simple-icons": "^7.2.0",
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-clipboard-manager": "^2",
     "@tauri-apps/plugin-dialog": "^2.5.0",