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/fo.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-fo" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="fo-a">
+      <path fill-opacity=".7" d="M-78 32h640v480H-78z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="0" clip-path="url(#fo-a)" transform="translate(78 -32)">
+    <path fill="#fff" d="M-78 32h663.9v480H-78z"/>
+    <path fill="#003897" d="M-76 218.7h185.9V32H216v186.7h371.8v106.6H216V512H109.9V325.3h-186z"/>
+    <path fill="#d72828" d="M-76 245.3h212.4V32h53.1v213.3H588v53.4H189.5V512h-53V298.7H-76z"/>
+  </g>
+</svg>