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/az.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-az" viewBox="0 0 640 480">
+  <path fill="#3f9c35" d="M.1 0h640v480H.1z"/>
+  <path fill="#ed2939" d="M.1 0h640v320H.1z"/>
+  <path fill="#00b9e4" d="M.1 0h640v160H.1z"/>
+  <circle cx="304" cy="240" r="72" fill="#fff"/>
+  <circle cx="320" cy="240" r="60" fill="#ed2939"/>
+  <path fill="#fff" d="m384 200 7.7 21.5 20.6-9.8-9.8 20.7L424 240l-21.5 7.7 9.8 20.6-20.6-9.8L384 280l-7.7-21.5-20.6 9.8 9.8-20.6L344 240l21.5-7.7-9.8-20.6 20.6 9.8z"/>
+</svg>