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/cv.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-cv" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="cv-a">
+      <path fill-opacity=".7" d="M-123.4 0h682.6v512h-682.6z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" clip-path="url(#cv-a)" transform="translate(115.7)scale(.94)">
+    <path fill="#fff" d="M-123.4 233H723v206h-846.5z"/>
+    <path fill="#081873" d="M-122.8 0h846v256.6h-846zm.3 385.9h852.1V512h-852.1z"/>
+    <path fill="#de3929" d="M-122.5 302.6h846v39.6h-846z"/>
+    <path fill="#ffce08" d="m131 399.2 6.6 20.4H159l-17.4 12.7 6.6 20.5L131 440l-17.4 12.7 6.7-20.5-17.4-12.7h21.5M317 250.4l6.7 20.5H345l-17.4 12.6 6.6 20.5-17.4-12.7-17.4 12.7 6.6-20.5-17.4-12.6h21.6m-222 64.4 6.6 20.5h21.5L99 368.6l6.7 20.4-17.4-12.6L70.9 389l6.6-20.4-17.4-12.7h21.5M317 329.5l6.7 20.4H345l-17.4 12.7 6.6 20.4-17.4-12.6-17.4 12.7 6.6-20.5-17.4-12.7h21.6m-40.5-161.7 6.7 20.4H298l-17.4 12.7 6.6 20.5-17.4-12.7-17.4 12.7 6.7-20.5-17.5-12.7h21.6m-64.5-45.2 6.7 20.5h21.5l-17.4 12.6 6.6 20.5-17.4-12.6-17.4 12.6 6.7-20.5-17.4-12.6H192m-64.5 2.9 6.7 20.5h21.5l-17.4 12.6 6.7 20.5-17.5-12.7-17.4 12.7 6.7-20.5-17.4-12.6H121m-34.8 43.2 6.6 20.5h21.6l-17.5 12.6 6.7 20.5-17.4-12.7-17.4 12.7 6.6-20.5L58 271h21.5m119.2 149.4 6.7 20.5h21.5l-17.4 12.6 6.7 20.5-17.5-12.7-17.4 12.7 6.7-20.5-17.4-12.6H192m82.2-41.7 6.6 20.4h21.5L285 432.3l6.7 20.5-17.4-12.7-17.5 12.7 6.7-20.5-17.4-12.7h21.5"/>
+  </g>
+</svg>