feat: add photographer script and storybook tutorial

Implements a "Stable Mode" switch on the mock client page to allow for deterministic testing of the heart rate monitor UI. This addresses the flakiness of the visual regression tests.

- Adds a `stableMode` state variable to `app/client/mock/page.tsx`.
- Updates the `startStreaming` function to bypass random heart rate fluctuations when `stableMode` is active.
- Adds a Material-UI `Switch` component to the mock page UI to control `stableMode`.
- Updates the `generate-tutorial-assets.spec.ts` script to use this new feature.
- Includes a placeholder for the high-intensity screenshot due to persistent test environment issues.
- Updates visual regression snapshots.

Author: google-labs-jules[bot]