feat: gradient buttons, language selector with flags, SVG support, and UI improvements

- Add linear gradient backgrounds to primary/secondary/danger buttons
- Add language selector with flag icons in ComboBox
- Add SVG support via egui_extras feature
- Add cancel confirmation dialog on exit
- Update Yali logo to SVG icon
- Add flag SVGs for all countries
- Improve users step layout with form helpers
- Refactor button API (ui::theme::*_button now takes ui param)

assets/flags/mu.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-mu" viewBox="0 0 640 480">
+  <g fill-rule="evenodd">
+    <path fill="#00a04d" d="M0 360h640v120H0z"/>
+    <path fill="#151f6d" d="M0 120h640v120H0z"/>
+    <path fill="#ee2737" d="M0 0h640v120H0z"/>
+    <path fill="#ffcd00" d="M0 240h640v120H0z"/>
+  </g>
+</svg>