refactor(frontend): EmptyState + LoadingSkeleton primitives; unified ThoughtForm replaces PostThoughtForm and ReplyForm

thoughts-frontend/components/empty-state.tsx

@@ -0,0 +1,12 @@
+interface EmptyStateProps {
+  message: string
+  className?: string
+}
+
+export function EmptyState({ message, className }: EmptyStateProps) {
+  return (
+    <p className={`text-center text-muted-foreground pt-8 ${className ?? ""}`}>
+      {message}
+    </p>
+  )
+}