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/in.svg

@@ -0,0 +1,25 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="flag-icons-in" viewBox="0 0 640 480">
+  <path fill="#f93" d="M0 0h640v160H0z"/>
+  <path fill="#fff" d="M0 160h640v160H0z"/>
+  <path fill="#128807" d="M0 320h640v160H0z"/>
+  <g transform="matrix(3.2 0 0 3.2 320 240)">
+    <circle r="20" fill="#008"/>
+    <circle r="17.5" fill="#fff"/>
+    <circle r="3.5" fill="#008"/>
+    <g id="in-d">
+      <g id="in-c">
+        <g id="in-b">
+          <g id="in-a" fill="#008">
+            <circle r=".9" transform="rotate(7.5 -8.8 133.5)"/>
+            <path d="M0 17.5.6 7 0 2l-.6 5z"/>
+          </g>
+          <use xlink:href="#in-a" width="100%" height="100%" transform="rotate(15)"/>
+        </g>
+        <use xlink:href="#in-b" width="100%" height="100%" transform="rotate(30)"/>
+      </g>
+      <use xlink:href="#in-c" width="100%" height="100%" transform="rotate(60)"/>
+    </g>
+    <use xlink:href="#in-d" width="100%" height="100%" transform="rotate(120)"/>
+    <use xlink:href="#in-d" width="100%" height="100%" transform="rotate(-120)"/>
+  </g>
+</svg>