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/cm.svg

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="flag-icons-cm" viewBox="0 0 640 480">
+  <path fill="#007a5e" d="M0 0h213.3v480H0z"/>
+  <path fill="#ce1126" d="M213.3 0h213.4v480H213.3z"/>
+  <path fill="#fcd116" d="M426.7 0H640v480H426.7z"/>
+  <g fill="#fcd116" transform="translate(320 240)scale(7.1111)">
+    <g id="cm-b">
+      <path id="cm-a" d="M0-8-2.5-.4 1.3.9z"/>
+      <use xlink:href="#cm-a" width="100%" height="100%" transform="scale(-1 1)"/>
+    </g>
+    <use xlink:href="#cm-b" width="100%" height="100%" transform="rotate(72)"/>
+    <use xlink:href="#cm-b" width="100%" height="100%" transform="rotate(144)"/>
+    <use xlink:href="#cm-b" width="100%" height="100%" transform="rotate(-144)"/>
+    <use xlink:href="#cm-b" width="100%" height="100%" transform="rotate(-72)"/>
+  </g>
+</svg>