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/cf.svg

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-cf" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="cf-a">
+      <path fill-opacity=".7" d="M-12.4 32h640v480h-640z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#cf-a)" transform="translate(12.4 -32)">
+    <path fill="#00f" d="M-52 32h719.3v119H-52z"/>
+    <path fill="#ff0" d="M-52 391.6h719.3V512H-52z"/>
+    <path fill="#009a00" d="M-52 271.3h719.3v120.3H-52z"/>
+    <path fill="#fff" d="M-52 151h719.3v120.3H-52z"/>
+    <path fill="red" d="M247.7 32.5h119.9V512H247.7z"/>
+    <path fill="#ff0" d="m99.3 137.7-31.5-21.8-31.3 22L47.4 101 16.9 78l38.2-1 12.5-36.3L80.3 77l38.1.7L88.2 101"/>
+  </g>
+</svg>