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/fm.svg

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-fm" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="fm-a">
+      <path fill-opacity=".7" d="M-81.3 0h682.6v512H-81.3z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="1pt" clip-path="url(#fm-a)" transform="translate(76.3)scale(.94)">
+    <path fill="#6797d6" d="M-252 0H772v512H-252z"/>
+    <path fill="#fff" d="m259.8 123-32.4 22.2 12.4-35.9-32.5-22.2h40.1l12.4-35.9 12.4 36h40l-32.4 22.1 12.4 35.9M259.8 390l-32.4-22.2 12.4 36-32.5 22.1h40.1l12.4 35.9 12.4-36 40 .1-32.4-22.2 12.4-35.9m-188.4-92.4L79.3 306l1.4-38-37.5-11.7 38.4-11.7 1.3-38 22.3 30.8 38.4-11.8-24.6 30.7 22.4 30.7m274.2-11.7 24.6 30.7-1.4-38 37.5-11.7-38.4-11.7-1.3-38-22.3 30.8-38.4-11.8 24.6 30.7-22.4 30.7"/>
+  </g>
+</svg>