feat(virtual-desktop): add usePrivateState hook for localStorage persistence

Provides OpenAI-style privateContent workaround using localStorage with
hostContext.toolInfo.id as the storage key. This allows UI state to persist
across sessions without being sent to the model.

Author: ochafik

examples/virtual-desktop-server/src/mcp-app.tsx

@@ -53,6 +53,50 @@ const log = {
   error: console.error.bind(console, "[VNC]"),
 };
 
+/**
+ * Private state persistence using localStorage + toolInfo.id.
+ * This is a workaround for OpenAI-style privateContent - state that persists
+ * across sessions but is NOT sent to the model.
+ */
+export function usePrivateState<T>(
+  toolId: string | undefined,
+  initialState: T,
+): [T, (state: T | ((prev: T) => T)) => void] {
+  const storageKey = toolId ? `mcp-app-private:${toolId}` : null;
+
+  const [state, setState] = useState<T>(() => {
+    if (!storageKey) return initialState;
+    try {
+      const stored = localStorage.getItem(storageKey);
+      return stored ? JSON.parse(stored) : initialState;
+    } catch {
+      return initialState;
+    }
+  });
+
+  const setStateAndPersist = useCallback(
+    (newState: T | ((prev: T) => T)) => {
+      setState((prev) => {
+        const next =
+          typeof newState === "function"
+            ? (newState as (prev: T) => T)(prev)
+            : newState;
+        if (storageKey) {
+          try {
+            localStorage.setItem(storageKey, JSON.stringify(next));
+          } catch (e) {
+            log.warn("Failed to persist private state:", e);
+          }
+        }
+        return next;
+      });
+    },
+    [storageKey],
+  );
+
+  return [state, setStateAndPersist];
+}
+
 // noVNC loading state
 let RFBClass: new (
   target: HTMLElement,