feat(phase-9): implement MVP polishing and closure

T-032: User management admin panel - backend users router with CRUD, frontend UsersPage with modals

T-033: Audit log viewer - backend audit router with filters/pagination, frontend AuditLogPage

T-034: Global error handling - ErrorBoundary, LoadingSpinner, ErrorMessage, Toast components

T-035: Backend tests - pytest setup with SQLite, tests for health/auth/techniques/tests

T-036: Documentation - Updated README with testing section, created docs/API.md

frontend/src/components/LoadingSpinner.tsx

@@ -0,0 +1,36 @@
+import { Loader2 } from "lucide-react";
+
+interface LoadingSpinnerProps {
+  size?: "sm" | "md" | "lg";
+  text?: string;
+  fullScreen?: boolean;
+}
+
+const sizeClasses = {
+  sm: "h-4 w-4",
+  md: "h-8 w-8",
+  lg: "h-12 w-12",
+};
+
+export default function LoadingSpinner({
+  size = "md",
+  text,
+  fullScreen = false,
+}: LoadingSpinnerProps) {
+  const content = (
+    <div className="flex flex-col items-center justify-center gap-3">
+      <Loader2 className={`animate-spin text-cyan-400 ${sizeClasses[size]}`} />
+      {text && <p className="text-sm text-gray-400">{text}</p>}
+    </div>
+  );
+
+  if (fullScreen) {
+    return (
+      <div className="fixed inset-0 flex items-center justify-center bg-gray-950/80 backdrop-blur-sm z-50">
+        {content}
+      </div>
+    );
+  }
+
+  return <div className="flex h-64 items-center justify-center">{content}</div>;
+}