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/gh.svg

@@ -0,0 +1,6 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-gh" viewBox="0 0 640 480">
+  <path fill="#006b3f" d="M0 0h640v480H0z"/>
+  <path fill="#fcd116" d="M0 0h640v320H0z"/>
+  <path fill="#ce1126" d="M0 0h640v160H0z"/>
+  <path fill="#000001" d="m320 160 52 160-136.1-98.9H404L268 320z"/>
+</svg>