fix(dashboard): stabilize cached snapshot references

Memoize cached dashboard snapshots so chart data stays referentially stable while live data loads. Add a regression test for cache-only parent rerenders to prevent the Recharts update loop when returning from canvas.

tests/use-dashboard-snapshot.test.ts

@@ -0,0 +1,119 @@
+/* @vitest-environment jsdom */
+
+import React, { act, useEffect } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+const useAuthQueryMock = vi.hoisted(() => vi.fn());
+const readDashboardSnapshotCacheMock = vi.hoisted(() => vi.fn());
+const writeDashboardSnapshotCacheMock = vi.hoisted(() => vi.fn());
+const clearDashboardSnapshotCacheMock = vi.hoisted(() => vi.fn());
+const getDashboardSnapshotCacheInvalidationSignalKeyMock = vi.hoisted(() => vi.fn());
+
+vi.mock("@/convex/_generated/api", () => ({
+  api: {
+    dashboard: { getSnapshot: "dashboard.getSnapshot" },
+  },
+}));
+
+vi.mock("@/hooks/use-auth-query", () => ({
+  useAuthQuery: useAuthQueryMock,
+}));
+
+vi.mock("@/lib/dashboard-snapshot-cache", () => ({
+  readDashboardSnapshotCache: readDashboardSnapshotCacheMock,
+  writeDashboardSnapshotCache: writeDashboardSnapshotCacheMock,
+  clearDashboardSnapshotCache: clearDashboardSnapshotCacheMock,
+  getDashboardSnapshotCacheInvalidationSignalKey:
+    getDashboardSnapshotCacheInvalidationSignalKeyMock,
+}));
+
+import { useDashboardSnapshot } from "@/hooks/use-dashboard-snapshot";
+
+const latestHookValue: {
+  current: ReturnType<typeof useDashboardSnapshot> | null;
+} = { current: null };
+
+function createCachedSnapshot() {
+  return {
+    balance: { available: 120 },
+    subscription: null,
+    usageStats: null,
+    recentTransactions: [
+      {
+        _id: "tx_1",
+        _creationTime: 1,
+        type: "usage",
+        status: "committed",
+        amount: -12,
+      },
+    ],
+    canvases: [],
+    mediaPreview: [],
+  };
+}
+
+function HookHarness({ userId }: { userId: string | null }) {
+  const value = useDashboardSnapshot(userId);
+
+  useEffect(() => {
+    latestHookValue.current = value;
+    return () => {
+      latestHookValue.current = null;
+    };
+  }, [value]);
+
+  return null;
+}
+
+(globalThis as typeof globalThis & { IS_REACT_ACT_ENVIRONMENT?: boolean }).IS_REACT_ACT_ENVIRONMENT = true;
+
+describe("useDashboardSnapshot", () => {
+  let container: HTMLDivElement | null = null;
+  let root: Root | null = null;
+
+  afterEach(async () => {
+    if (root) {
+      await act(async () => {
+        root?.unmount();
+      });
+    }
+
+    container?.remove();
+    container = null;
+    root = null;
+    latestHookValue.current = null;
+    useAuthQueryMock.mockReset();
+    readDashboardSnapshotCacheMock.mockReset();
+    writeDashboardSnapshotCacheMock.mockReset();
+    clearDashboardSnapshotCacheMock.mockReset();
+    getDashboardSnapshotCacheInvalidationSignalKeyMock.mockReset();
+  });
+
+  it("keeps the cached snapshot reference stable across parent rerenders", async () => {
+    useAuthQueryMock.mockReturnValue(undefined);
+    getDashboardSnapshotCacheInvalidationSignalKeyMock.mockReturnValue("dashboard:invalidate");
+    readDashboardSnapshotCacheMock.mockImplementation(() => ({
+      snapshot: createCachedSnapshot(),
+    }));
+
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = createRoot(container);
+
+    await act(async () => {
+      root?.render(React.createElement(HookHarness, { userId: "user_1" }));
+    });
+
+    const firstSnapshot = latestHookValue.current?.snapshot;
+
+    await act(async () => {
+      root?.render(React.createElement(HookHarness, { userId: "user_1" }));
+    });
+
+    const secondSnapshot = latestHookValue.current?.snapshot;
+
+    expect(latestHookValue.current?.source).toBe("cache");
+    expect(firstSnapshot).toBe(secondSnapshot);
+  });
+});