feat: add save screenshot to disk for iOS device

[mlikasam-askui]

Summary

Adds the ability to save a device screenshot directly to a file on disk via a new MCP tool and a new IOSDevice method.

Changes

• IOSDevice.save_screenshot_to_disk(absolute_image_path) (in ios_device.py):
  • Takes a path (str or Path); resolves to an absolute path.
  • Requires path to end with .png.
  • Fails if the file already exists (no overwrite).
  • Ensures device is booted before any I/O; creates parent directories as needed; takes screenshot and saves as PNG.
• MCP tool ios_save_screenshot_to_disk(absolute_image_path: str) (in server.py):
  • Calls the selected device’s save_screenshot_to_disk and returns a confirmation message.
  • Docstring clarifies .png requirement, parent-dir creation, and no-overwrite behavior.

Rationale

Enables workflows that need screenshots persisted to a known path (e.g. for reports, debugging, or downstream tools) without going through the in-memory screenshot tool and manual file handling.