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/ly.svg

@@ -0,0 +1,13 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-ly" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="ly-a">
+      <path d="M166.7-20h666.6v500H166.7z"/>
+    </clipPath>
+  </defs>
+  <g clip-path="url(#ly-a)" transform="matrix(.96 0 0 .96 -160 19.2)">
+    <path fill="#239e46" d="M0-20h1000v500H0z"/>
+    <path fill="#000001" d="M0-20h1000v375H0z"/>
+    <path fill="#e70013" d="M0-20h1000v125H0z"/>
+    <path fill="#fff" d="M544.2 185.8a54.3 54.3 0 1 0 0 88.4 62.5 62.5 0 1 1 0-88.4M530.4 230l84.1-27.3-52 71.5v-88.4l52 71.5z"/>
+  </g>
+</svg>