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/dk.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-dk" viewBox="0 0 640 480">
+  <path fill="#c8102e" d="M0 0h640.1v480H0z"/>
+  <path fill="#fff" d="M205.7 0h68.6v480h-68.6z"/>
+  <path fill="#fff" d="M0 205.7h640.1v68.6H0z"/>
+</svg>