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/gg.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="flag-icons-gg" viewBox="0 0 640 480">
+  <path fill="#fff" d="M0 0h640v480H0z"/>
+  <path fill="#e8112d" d="M256 0h128v480H256z"/>
+  <path fill="#e8112d" d="M0 176h640v128H0z"/>
+  <path id="gg-a" fill="#f9dd16" d="m110 286.7 23.3-23.4h210v-46.6h-210L110 193.3z"/>
+  <use xlink:href="#gg-a" width="36" height="24" transform="rotate(90 320 240)"/>
+  <use xlink:href="#gg-a" width="36" height="24" transform="rotate(-90 320 240)"/>
+  <use xlink:href="#gg-a" width="36" height="24" transform="rotate(180 320 240)"/>
+</svg>