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/sc.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-sc" viewBox="0 0 640 480">
+  <path fill="#fff" d="M0 0h640v480H0Z"/>
+  <path fill="#d92223" d="M0 480V0h640v160z"/>
+  <path fill="#fcd955" d="M0 480V0h426.7z"/>
+  <path fill="#003d88" d="M0 480V0h213.3z"/>
+  <path fill="#007a39" d="m0 480 640-160v160z"/>
+</svg>