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/xx.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-xx" viewBox="0 0 640 480">
+  <path fill="#fff" fill-rule="evenodd" stroke="#adb5bd" stroke-width="1.1" d="M.5.5h638.9v478.9H.5z"/>
+  <path fill="none" stroke="#adb5bd" stroke-width="1.1" d="m.5.5 639 479m0-479-639 479"/>
+</svg>