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/th.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-th" viewBox="0 0 640 480">
+  <g fill-rule="evenodd">
+    <path fill="#f4f5f8" d="M0 0h640v480H0z"/>
+    <path fill="#2d2a4a" d="M0 162.5h640v160H0z"/>
+    <path fill="#a51931" d="M0 0h640v82.5H0zm0 400h640v80H0z"/>
+  </g>
+</svg>