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/ch.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-ch" viewBox="0 0 640 480">
+  <g fill-rule="evenodd" stroke-width="1pt">
+    <path fill="red" d="M0 0h640v480H0z"/>
+    <g fill="#fff">
+      <path d="M170 195h300v90H170z"/>
+      <path d="M275 90h90v300h-90z"/>
+    </g>
+  </g>
+</svg>