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/vc.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-vc" viewBox="0 0 640 480">
+  <g fill-rule="evenodd">
+    <path fill="#f4f100" d="M0 0h640v480H0z"/>
+    <path fill="#199a00" d="M490 0h150v480H490z"/>
+    <path fill="#0058aa" d="M0 0h150v480H0z"/>
+    <path fill="#199a00" d="m259.3 130-46.4 71.3 44.7 74.4 43.8-73.7zm121.2 0-46.3 71.3 44.7 74.4 43.8-73.7zm-61.2 97.3-46.4 71.4 44.8 74.4 43.8-73.7-42.2-72z"/>
+  </g>
+</svg>