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/es-ct.svg

@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-es-ct" viewBox="0 0 640 480">
+  <path fill="#fcdd09" d="M0 0h640v480H0z"/>
+  <path stroke="#da121a" stroke-width="60" d="M0 90h810m0 120H0m0 120h810m0 120H0" transform="scale(.79012 .88889)"/>
+</svg>