Relicense from CC0 to a source-available license

Replaces the CC0 1.0 dedication with the Screendrop Source-Available
License: personal and internal use, building, and modification are
allowed; selling, redistributing (source or compiled builds),
rebranding, and offering it as a commercial product or service require
written permission. Adds a contributor clause so PRs are licensed to
the copyright holder.

Note this is source-available, not OSI open source, and it only binds
copies obtained from this commit forward.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SfEwCf4FVyhY6qZceoKUTa