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/Toast.tsx

@@ -0,0 +1,88 @@
+import { useState, useEffect, createContext, useContext, useCallback, type ReactNode } from "react";
+import { CheckCircle, AlertCircle, Info, X } from "lucide-react";
+
+type ToastType = "success" | "error" | "info";
+
+interface Toast {
+  id: string;
+  type: ToastType;
+  message: string;
+}
+
+interface ToastContextValue {
+  showToast: (type: ToastType, message: string) => void;
+}
+
+const ToastContext = createContext<ToastContextValue | undefined>(undefined);
+
+export function useToast() {
+  const context = useContext(ToastContext);
+  if (!context) {
+    throw new Error("useToast must be used within a ToastProvider");
+  }
+  return context;
+}
+
+const icons = {
+  success: CheckCircle,
+  error: AlertCircle,
+  info: Info,
+};
+
+const colors = {
+  success: "border-green-500/30 bg-green-900/30 text-green-400",
+  error: "border-red-500/30 bg-red-900/30 text-red-400",
+  info: "border-cyan-500/30 bg-cyan-900/30 text-cyan-400",
+};
+
+export function ToastProvider({ children }: { children: ReactNode }) {
+  const [toasts, setToasts] = useState<Toast[]>([]);
+
+  const showToast = useCallback((type: ToastType, message: string) => {
+    const id = Math.random().toString(36).substr(2, 9);
+    setToasts((prev) => [...prev, { id, type, message }]);
+  }, []);
+
+  const removeToast = useCallback((id: string) => {
+    setToasts((prev) => prev.filter((toast) => toast.id !== id));
+  }, []);
+
+  return (
+    <ToastContext.Provider value={{ showToast }}>
+      {children}
+      <div className="fixed bottom-4 right-4 z-50 flex flex-col gap-2">
+        {toasts.map((toast) => (
+          <ToastItem
+            key={toast.id}
+            toast={toast}
+            onClose={() => removeToast(toast.id)}
+          />
+        ))}
+      </div>
+    </ToastContext.Provider>
+  );
+}
+
+function ToastItem({ toast, onClose }: { toast: Toast; onClose: () => void }) {
+  const Icon = icons[toast.type];
+
+  useEffect(() => {
+    const timer = setTimeout(onClose, 5000);
+    return () => clearTimeout(timer);
+  }, [onClose]);
+
+  return (
+    <div
+      className={`flex items-center gap-3 rounded-lg border px-4 py-3 shadow-lg animate-in slide-in-from-right ${colors[toast.type]}`}
+    >
+      <Icon className="h-5 w-5 flex-shrink-0" />
+      <p className="text-sm">{toast.message}</p>
+      <button
+        onClick={onClose}
+        className="ml-2 rounded p-1 hover:bg-white/10"
+      >
+        <X className="h-4 w-4" />
+      </button>
+    </div>
+  );
+}