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/st.svg

@@ -0,0 +1,16 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" id="flag-icons-st" viewBox="0 0 640 480">
+  <path fill="#12ad2b" d="M0 0h640v480H0z"/>
+  <path fill="#ffce00" d="M0 137.1h640V343H0z"/>
+  <path fill="#d21034" d="M0 0v480l240-240"/>
+  <g id="st-c" transform="translate(351.6 240)scale(.34286)">
+    <g id="st-b">
+      <path id="st-a" fill="#000001" d="M0-200V0h100" transform="rotate(18 0 -200)"/>
+      <use xlink:href="#st-a" width="100%" height="100%" transform="scale(-1 1)"/>
+    </g>
+    <use xlink:href="#st-b" width="100%" height="100%" transform="rotate(72)"/>
+    <use xlink:href="#st-b" width="100%" height="100%" transform="rotate(144)"/>
+    <use xlink:href="#st-b" width="100%" height="100%" transform="rotate(-144)"/>
+    <use xlink:href="#st-b" width="100%" height="100%" transform="rotate(-72)"/>
+  </g>
+  <use xlink:href="#st-c" width="100%" height="100%" x="700" transform="translate(-523.2)"/>
+</svg>