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/cefta.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-cefta" viewBox="0 0 640 480">
+  <path fill="#039" d="M0 0h640v480H0z"/>
+  <circle cx="320" cy="249.8" r="30.4" fill="none" stroke="#fc0" stroke-width="27.5"/>
+  <circle cx="320" cy="249.8" r="88.3" fill="none" stroke="#fc0" stroke-width="27.5"/>
+  <path fill="#039" d="m404.7 165.1 84.7 84.7-84.7 84.7-84.7-84.7z"/>
+  <path fill="#fc0" d="M175.7 236.1h59.2v27.5h-59.2zm259.1 0h88.3v27.5h-88.3zM363 187.4l38.8-38.8 19.4 19.5-38.7 38.7zM306.3 48.6h27.5v107.1h-27.5z"/>
+  <circle cx="225.1" cy="159.6" r="13.7" fill="#fc0"/>
+  <circle cx="144.3" cy="249.8" r="13.7" fill="#fc0"/>
+  <circle cx="320" cy="381.4" r="13.7" fill="#fc0"/>
+  <circle cx="320" cy="425.5" r="13.7" fill="#fc0"/>
+  <circle cx="408.3" cy="249.8" r="13.7" fill="#fc0"/>
+  <path fill="#fc0" d="m208.3 341.5 19.5-19.4 19.4 19.4-19.4 19.5zm204.7 21 19.5-19.5 19.5 19.5-19.5 19.4z"/>
+</svg>