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/lt.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-lt" viewBox="0 0 640 480">
+  <g fill-rule="evenodd" stroke-width="1pt" transform="scale(.64143 .96773)">
+    <rect width="1063" height="708.7" fill="#006a44" rx="0" ry="0" transform="scale(.93865 .69686)"/>
+    <rect width="1063" height="236.2" y="475.6" fill="#c1272d" rx="0" ry="0" transform="scale(.93865 .69686)"/>
+    <path fill="#fdb913" d="M0 0h997.8v164.6H0z"/>
+  </g>
+</svg>