docs: publish fresh v1.2 screenshots

[jakthom]

Summary

• capture the current dashboard, connector editor, and API reference from a freshly started Beacon instance
• publish the images under v1.2.0-specific filenames
• update README image links so GitHub cannot serve the stale pre-redesign assets

Verification

• confirmed the live source, both sinks, and both connector routes were up before capture
• visually inspected all three committed screenshots
• confirmed every image is a 1280x720 PNG
• ran git diff --check