Refresh README media for Decoy Mode

[alex-w-developer]

Summary

• replace the pre-Decoy popup screenshot with a clean capture of the merged UI
• replace the personal-browser recording with a compact, isolated-profile demo of normal and Decoy modes
• update README headings, dimensions, and alt text for the new media

Why

The existing screenshot no longer matched the popup after PR #7, and the existing GIF showed unrelated personal browser state. The replacement assets were captured from the automated browser-test profile and contain only GetBlocked! UI.

Validation

• npm run check
• npm run test:browser — 17 passed during clean asset capture
• visual inspection of the normal and Decoy frames
• GIF metadata: 356×707, 3 frames
• git diff --check

Closes #9