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/sg.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-sg" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="sg-a">
+      <path fill-opacity=".7" d="M0 0h640v480H0z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#sg-a)">
+    <path fill="#fff" d="M-20 0h720v480H-20z"/>
+    <path fill="#df0000" d="M-20 0h720v240H-20z"/>
+    <path fill="#fff" d="M146 40.2a84.4 84.4 0 0 0 .8 165.2 86 86 0 0 1-106.6-59 86 86 0 0 1 59-106c16-4.6 30.8-4.7 46.9-.2z"/>
+    <path fill="#fff" d="m133 110 4.9 15-13-9.2-12.8 9.4 4.7-15.2-12.8-9.3 15.9-.2 5-15 5 15h15.8zm17.5 52 5 15.1-13-9.2-12.9 9.3 4.8-15.1-12.8-9.4 15.9-.1 4.9-15.1 5 15h16zm58.5-.4 4.9 15.2-13-9.3-12.8 9.3 4.7-15.1-12.8-9.3 15.9-.2 5-15 5 15h15.8zm17.4-51.6 4.9 15.1-13-9.2-12.8 9.3 4.8-15.1-12.9-9.4 16-.1 4.8-15.1 5 15h16zm-46.3-34.3 5 15.2-13-9.3-12.9 9.4 4.8-15.2-12.8-9.4 15.8-.1 5-15.1 5 15h16z"/>
+  </g>
+</svg>