Fix About modal links and improve Linux integration

- Use Tauri opener plugin for all external links in About modal
- Convert anchor tags to buttons with openUrl() for proper browser opening
- Add GTK_USE_PORTAL=1 for native file dialogs on Linux (KDE Plasma)
- Add Tux.svg and mexico-flag.svg for About modal signature

src-tauri/src/main.rs

@@ -2,5 +2,10 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")]
 
 fn main() {
+    // Use xdg-desktop-portal for file dialogs on Linux
+    // This makes GTK apps use native file pickers (e.g., KDE's on Plasma)
+    #[cfg(target_os = "linux")]
+    std::env::set_var("GTK_USE_PORTAL", "1");
+
     qbz_nix_lib::run()
 }