Remove README showcase placeholder

[yvette-carlisle]

Summary

• Remove the bare site URL and logo image from the README centered header.
• Leave the rsnap-style top section as title, description, and badges only until Decodex has real showcase screenshots or recordings.

Verification

• git diff --check

Semantic drift

• Not required: this only removes a visual placeholder and does not change behavior, commands, status text, or config claims.