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/bs.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-bs" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="bs-a">
+      <path fill-opacity=".7" d="M-12 0h640v480H-12z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#bs-a)" transform="translate(12)">
+    <path fill="#fff" d="M968.5 480h-979V1.8h979z"/>
+    <path fill="#ffe900" d="M968.5 344.5h-979V143.3h979z"/>
+    <path fill="#08ced6" d="M968.5 480h-979V320.6h979zm0-318.7h-979V2h979z"/>
+    <path fill="#000001" d="M-11 0c2.3 0 391.8 236.8 391.8 236.8L-12 479.2z"/>
+  </g>
+</svg>