docs: finalize Ptylon public screenshots

[alexfrmn]

Summary

• rename and regenerate the public showcase PNGs under the Ptylon identity
• align README and showcase-capture references; correct the navigation table
• replace the two remaining public-facing “Web Console” product references

Verification

• pnpm exec tsc --noEmit
• pnpm lint
• pnpm test:pty-gateway
• pnpm build
• isolated pnpm capture:showcase
• full-history gitleaks detect --log-opts=--all (20 commits; no leaks)

pnpm audit --prod --audit-level=high reports 19 existing low/moderate findings and no high findings.