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/lr.svg

@@ -0,0 +1,14 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-lr" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="lr-a">
+      <path fill-opacity=".7" d="M0 0h682.7v512H0z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#lr-a)" transform="scale(.9375)">
+    <path fill="#fff" d="M0 0h767.9v512H0z"/>
+    <path fill="#006" d="M0 0h232.7v232.8H0z"/>
+    <path fill="#c00" d="M0 464.9h767.9V512H0z"/>
+    <path fill="#c00" d="M0 465.4h767.9V512H0zm0-92.9h767.9v46.2H0zm0-93.2h766V326H0zM232.7 0h535.1v46.5H232.7zm0 186h535.1v46.8H232.7zm0-92.7h535.1v46.5H232.7z"/>
+    <path fill="#fff" d="m166.3 177.5-50.7-31-50.4 31.3 18.7-50.9-50.3-31.4 62.3-.4 19.3-50.7L135 95h62.3l-50.1 31.7z"/>
+  </g>
+</svg>