fix: address Storybook review findings

src/lib/ui/components/Button.svelte

@@ -1,6 +1,16 @@
 <script lang="ts">
+  import type { HTMLButtonAttributes } from "svelte/elements";
   import IconGlyph from "./IconGlyph.svelte";
 
+  type ButtonProps = Omit<HTMLButtonAttributes, "type"> & {
+    label: string;
+    variant?: "primary" | "secondary" | "danger" | "ghost";
+    size?: "default" | "compact";
+    icon?: string;
+    loading?: boolean;
+    type?: "button" | "submit" | "reset";
+  };
+
   let {
     label,
     variant = "primary",
@@ -9,19 +19,14 @@
     disabled = false,
     loading = false,
     type = "button",
-  }: {
-    label: string;
-    variant?: "primary" | "secondary" | "danger" | "ghost";
-    size?: "default" | "compact";
-    icon?: string;
-    disabled?: boolean;
-    loading?: boolean;
-    type?: "button" | "submit" | "reset";
-  } = $props();
+    class: className = "",
+    ...buttonProps
+  }: ButtonProps = $props();
 </script>
 
 <button
-  class="ui-button"
+  {...buttonProps}
+  class={className ? `ui-button ${className}` : "ui-button"}
   data-variant={variant}
   data-size={size}
   data-loading={loading}