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/jp.svg

@@ -0,0 +1,11 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-jp" viewBox="0 0 640 480">
+  <defs>
+    <clipPath id="jp-a">
+      <path fill-opacity=".7" d="M-88 32h640v480H-88z"/>
+    </clipPath>
+  </defs>
+  <g fill-rule="evenodd" stroke-width="1pt" clip-path="url(#jp-a)" transform="translate(88 -32)">
+    <path fill="#fff" d="M-128 32h720v480h-720z"/>
+    <circle cx="523.1" cy="344.1" r="194.9" fill="#bc002d" transform="translate(-168.4 8.6)scale(.76554)"/>
+  </g>
+</svg>