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/pa.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-pa" viewBox="0 0 640 480">
+  <path fill="#fff" d="M0 0h640v480H0z"/>
+  <path fill="#db0000" d="M323 0h358v225.3H323z"/>
+  <path fill="#0000ab" d="M0 225.3h323.1V480H0z"/>
+  <path fill="#0000ab" fill-rule="evenodd" d="m214.8 177.7-42-29.4-41.7 29.6 15.5-48L105 100l51.6-.4 16-48 16.3 47.9h51.6l-41.5 30 15.9 48z"/>
+  <path fill="#d80000" fill-rule="evenodd" d="m516.9 413.9-42.4-27.7-42.1 28 15.6-45.6-42-28 52-.5 16.2-45.4 16.4 45.3h52l-41.8 28.5 16 45.4z"/>
+</svg>