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/ss.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-ss" viewBox="0 0 640 480">
+  <path fill="#078930" d="M0 336h640v144H0z"/>
+  <path fill="#fff" d="M0 144h640v192H0z"/>
+  <path fill="#000001" d="M0 0h640v144H0z"/>
+  <path fill="#da121a" d="M0 168h640v144H0z"/>
+  <path fill="#0f47af" d="m0 0 415.7 240L0 480z"/>
+  <path fill="#fcdd09" d="M200.7 194.8 61.7 240l139 45.1L114.9 167v146z"/>
+</svg>