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/pk.svg

@@ -0,0 +1,15 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-pk" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="pk-a">
+      <path fill-opacity=".7" d="M-52.3 0h682.6v512H-52.3z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="1pt" clip-path="url(#pk-a)" transform="translate(49)scale(.9375)">
+    <path fill="#0c590b" d="M-95 0h768v512H-95z"/>
+    <path fill="#fff" d="M-95 0H97.5v512H-95z"/>
+    <g fill="#fff">
+      <path d="m403.7 225.4-31.2-6.6-16.4 27.3-3.4-31.6-31-7.2 29-13-2.7-31.7 21.4 23.6 29.3-12.4-15.9 27.6 21 24z"/>
+      <path d="M415.4 306a121 121 0 0 1-161.3 59.4 122 122 0 0 1-59.5-162.1A119 119 0 0 1 266 139a156 156 0 0 0-11.8 10.9A112.3 112.3 0 0 0 415.5 306z"/>
+    </g>
+  </g>
+</svg>