Add sanitized product screenshots to the website and GitHub

[thsnkhn]

Problem

The README has one screenshot from #1, but users still cannot see enough of Harbor before downloading it. The website and GitHub need broader product coverage.

Goal

Show the current Harbor experience without exposing personal download history.

Scope

Add sanitized screenshots for:

• main downloads window
• torrent file selection
• media preview and format selection
• download details and activity
• Settings and traffic profiles

Acceptance criteria

• The website includes a clear screenshot gallery or product section.
• The README includes a compact set of current screenshots.
• Screenshots use demo URLs, filenames, destinations, and activity history.
• No personal paths, usernames, tokens, or real download history are visible.
• Images cover both light and dark appearance where useful.
• The screenshots match the current released interface.

Related

Follow-up to #1, which added the first README screenshot.