Fix README demo GIF framing

[afurm]

Summary

• Replaces the README demo GIF with a corrected 16:9 recording.
• Removes the gray pillarbox band caused by recording a 16:10 viewport into a 16:9 video frame.
• Keeps the README text unchanged from PR Strengthen README product hook #47.

Verification

• Re-ran Draftmora locally with a temporary SQLite demo database.
• Re-recorded the app flow with Playwright using a matching 1600x900 viewport and video frame.
• Converted the recording to .github/assets/draftmora-demo.gif at 960x540.
• Confirmed the first frame has no gray side band and ffprobe reports a 960x540 GIF.
• Ran git diff --check.