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/is.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-is" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="is-a">
+      <path fill-opacity=".7" d="M0 0h640v480H0z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="0" clip-path="url(#is-a)">
+    <path fill="#003897" d="M0 0h666.7v480H0z"/>
+    <path fill="#fff" d="M0 186.7h186.7V0h106.6v186.7h373.4v106.6H293.3V480H186.7V293.3H0z"/>
+    <path fill="#d72828" d="M0 213.3h213.3V0h53.4v213.3h400v53.4h-400V480h-53.4V266.7H0z"/>
+  </g>
+</svg>