Rename Onyx theme to Black and Gold

Update displayed theme name from "Onyx" to "Black and Gold" in the
settings UI so the theme matches the requested branding. This change
ensures users see the new theme name in the theme selector and aligns
the UI with the prompt to call the theme Black and Gold.

apps/tauri/src/App.svelte

@@ -14,7 +14,7 @@
 </script>
 
 <div class={app.isDark ? "dark" : ""} data-theme={app.currentTheme ?? ""}>
-  <div class="h-screen w-screen" class:p-2={isLinux}>
+  <div class="h-screen w-screen" class:p-2={isLinux} class:onyx-border={app.currentTheme === "onyx"}>
     <div
       class="relative h-full w-full overflow-hidden bg-surface-light text-text-light dark:bg-surface-dark dark:text-text-dark"
       class:rounded-xl={isLinux}

apps/tauri/src/lib/screens/SettingsScreen.svelte

@@ -259,7 +259,7 @@
       <option value="nord">Nord</option>
       <option value="dracula">Dracula</option>
       <option value="solarized">Solarized Dark</option>
-      <option value="onyx">Onyx</option>
+      <option value="onyx">Black and Gold</option>
     </select>
   </section>