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/mv.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-mv" viewBox="0 0 640 480">
+  <path fill="#d21034" d="M0 0h640v480H0z"/>
+  <path fill="#007e3a" d="M120 120h400v240H120z"/>
+  <circle cx="350" cy="240" r="80" fill="#fff"/>
+  <circle cx="380" cy="240" r="80" fill="#007e3a"/>
+</svg>