docs(plugin): add light and dark registry screenshots

[dviejokfs]

Summary

• Add real light/dark screenshots captured from the plugin running locally in Temps.
• Declare repository-relative screenshot paths, accessible alt text, and captions in the temps manifest.
• Explain screenshot customization and privacy requirements in the README.

Verification

• 11 Bun tests passed, including PNG existence/signature/size checks.
• TypeScript check passed.
• Screenshots inspected: plugin UI only, no account details or credentials.

Publication

After merge, refresh the gotempsh/plugins Plugin catalog workflow. Its generated catalog pins the screenshot URLs to the plugin commit; registry.temps.sh consumes that catalog.