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/kr.svg

@@ -0,0 +1,24 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="flag-icons-kr" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="kr-a">
+      <path fill-opacity=".7" d="M-95.8-.4h682.7v512H-95.8z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#kr-a)" transform="translate(89.8 .4)scale(.9375)">
+    <path fill="#fff" d="M-95.8-.4H587v512H-95.8Z"/>
+    <g transform="rotate(-56.3 361.6 -101.3)scale(10.66667)">
+      <g id="kr-c">
+        <path id="kr-b" fill="#000001" d="M-6-26H6v2H-6Zm0 3H6v2H-6Zm0 3H6v2H-6Z"/>
+        <use xlink:href="#kr-b" width="100%" height="100%" y="44"/>
+      </g>
+      <path stroke="#fff" d="M0 17v10"/>
+      <path fill="#cd2e3a" d="M0-12a12 12 0 0 1 0 24Z"/>
+      <path fill="#0047a0" d="M0-12a12 12 0 0 0 0 24A6 6 0 0 0 0 0Z"/>
+      <circle cy="-6" r="6" fill="#cd2e3a"/>
+    </g>
+    <g transform="rotate(-123.7 191.2 62.2)scale(10.66667)">
+      <use xlink:href="#kr-c" width="100%" height="100%"/>
+      <path stroke="#fff" d="M0-23.5v3M0 17v3.5m0 3v3"/>
+    </g>
+  </g>
+</svg>