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/ve.svg

@@ -0,0 +1,26 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="flag-icons-ve" viewBox="0 0 640 480">
+  <defs>
+    <g id="ve-d" transform="translate(0 -36)">
+      <g id="ve-c">
+        <g id="ve-b">
+          <path id="ve-a" fill="#fff" d="M0-5-1.5-.2l2.8.9z"/>
+          <use xlink:href="#ve-a" width="180" height="120" transform="scale(-1 1)"/>
+        </g>
+        <use xlink:href="#ve-b" width="180" height="120" transform="rotate(72)"/>
+      </g>
+      <use xlink:href="#ve-b" width="180" height="120" transform="rotate(-72)"/>
+      <use xlink:href="#ve-c" width="180" height="120" transform="rotate(144)"/>
+    </g>
+  </defs>
+  <path fill="#cf142b" d="M0 0h640v480H0z"/>
+  <path fill="#00247d" d="M0 0h640v320H0z"/>
+  <path fill="#fc0" d="M0 0h640v160H0z"/>
+  <g id="ve-f" transform="matrix(4 0 0 4 320 336)">
+    <g id="ve-e">
+      <use xlink:href="#ve-d" width="180" height="120" transform="rotate(10)"/>
+      <use xlink:href="#ve-d" width="180" height="120" transform="rotate(30)"/>
+    </g>
+    <use xlink:href="#ve-e" width="180" height="120" transform="rotate(40)"/>
+  </g>
+  <use xlink:href="#ve-f" width="180" height="120" transform="rotate(-80 320 336)"/>
+</svg>