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/gr.svg

@@ -0,0 +1,16 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-gr" viewBox="0 0 640 480">
+  <path fill="#0d5eaf" fill-rule="evenodd" d="M0 0h640v53.3H0z"/>
+  <path fill="#fff" fill-rule="evenodd" d="M0 53.3h640v53.4H0z"/>
+  <path fill="#0d5eaf" fill-rule="evenodd" d="M0 106.7h640V160H0z"/>
+  <path fill="#fff" fill-rule="evenodd" d="M0 160h640v53.3H0z"/>
+  <path fill="#0d5eaf" d="M0 0h266.7v266.7H0z"/>
+  <path fill="#0d5eaf" fill-rule="evenodd" d="M0 213.3h640v53.4H0z"/>
+  <path fill="#fff" fill-rule="evenodd" d="M0 266.7h640V320H0z"/>
+  <path fill="#0d5eaf" fill-rule="evenodd" d="M0 320h640v53.3H0z"/>
+  <path fill="#fff" fill-rule="evenodd" d="M0 373.3h640v53.4H0z"/>
+  <g fill="#fff" fill-rule="evenodd" stroke-width="1.3">
+    <path d="M106.7 0H160v266.7h-53.3z"/>
+    <path d="M0 106.7h266.7V160H0z"/>
+  </g>
+  <path fill="#0d5eaf" d="M0 426.7h640V480H0z"/>
+</svg>