Add basic download handling for files

[culpen90]

Why

Quartz currently focuses on page navigation, but a browser also needs basic file download behavior. Links that trigger downloads should not feel like failed navigations, and users should be able to choose where files go.

Where to start

• Sources/Quartz/QuartzApp.swift
• WKNavigationDelegate download-related APIs
• WKDownloadDelegate

Suggested approach

Add minimal native download support. A good first version can show an NSSavePanel when WebKit starts a download, save the file to the selected location, and show success/failure feedback. A more complete version can add progress UI later.

Keep the initial scope small and reliable.

Acceptance criteria

• Downloads triggered by navigation or link clicks are handled by Quartz instead of appearing as broken page loads.
• Users can choose the destination path with a native save panel.
• Failed downloads show a useful native error message.
• Successful downloads leave the file at the selected destination.
• swift build succeeds.

Testing

• Download a small file from a normal website.
• Cancel the save panel and confirm Quartz handles cancellation cleanly.
• Download to a selected folder and confirm the file is written.