feat(frontend): add Skeleton primitive with scanline-shimmer

src/components/ui/skeleton.tsx

@@ -0,0 +1,59 @@
+import * as React from "react";
+import { cn } from "@/lib/utils";
+
+type SkeletonVariant = "default" | "text" | "circle" | "row";
+
+type SkeletonProps = {
+  className?: string;
+  variant?: SkeletonVariant;
+  width?: string | number;
+  height?: string | number;
+};
+
+const variantClass: Record<SkeletonVariant, string> = {
+  default: "rounded-md",
+  text: "rounded-sm h-3 w-full",
+  circle: "rounded-full",
+  row: "rounded-md h-9 w-full",
+};
+
+/**
+ * Hairline loading primitive.
+ * Voice: not a generic gray pulse — a 1px hairline ring with a slow
+ * horizontal scanline that sweeps the surface, like a precision
+ * instrument's sensor arm.
+ */
+export function Skeleton({
+  className,
+  variant = "default",
+  width,
+  height,
+}: SkeletonProps) {
+  const style: React.CSSProperties = {
+    ...(width !== undefined ? { width } : null),
+    ...(height !== undefined ? { height } : null),
+  };
+
+  return (
+    <div
+      className={cn(
+        "relative overflow-hidden bg-muted/30",
+        "ring-1 ring-inset ring-border/40",
+        variantClass[variant],
+        className,
+      )}
+      style={style}
+      aria-busy="true"
+      aria-live="polite"
+    >
+      <div
+        aria-hidden
+        className="absolute inset-y-0 left-0 w-1/2 animate-shimmer"
+        style={{
+          background:
+            "linear-gradient(90deg, transparent 0%, hsl(var(--muted-foreground) / 0.08) 50%, transparent 100%)",
+        }}
+      />
+    </div>
+  );
+}