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/tz.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-tz" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="tz-a">
+      <path fill-opacity=".7" d="M10 0h160v120H10z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="1pt" clip-path="url(#tz-a)" transform="matrix(4 0 0 4 -40 0)">
+    <path fill="#09f" d="M0 0h180v120H0z"/>
+    <path fill="#090" d="M0 0h180L0 120z"/>
+    <path fill="#000001" d="M0 120h40l140-95V0h-40L0 95z"/>
+    <path fill="#ff0" d="M0 91.5 137.2 0h13.5L0 100.5zM29.3 120 180 19.5v9L42.8 120z"/>
+  </g>
+</svg>