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/ax.svg

@@ -0,0 +1,18 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-ax" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="ax-a">
+      <path fill-opacity=".7" d="M106.3 0h1133.3v850H106.3z"/>
+    </clipPath>
+  </defs>
+  <g clip-path="url(#ax-a)" transform="matrix(.56472 0 0 .56482 -60 -.1)">
+    <path fill="#0053a5" d="M0 0h1300v850H0z"/>
+    <g fill="#ffce00">
+      <path d="M400 0h250v850H400z"/>
+      <path d="M0 300h1300v250H0z"/>
+    </g>
+    <g fill="#d21034">
+      <path d="M475 0h100v850H475z"/>
+      <path d="M0 375h1300v100H0z"/>
+    </g>
+  </g>
+</svg>