style: refresh landing social image and favicon

[jb-thery]

Summary

• Refresh the landing Open Graph PNG from a clean desktop hero screenshot.
• Replace the favicon with a black-background white Inter-style R monogram.
• Update the unused OG SVG fallback copy so public assets stay consistent.

Validation

• pnpm lint
• PUBLIC_RAGMIR_LANDING_URL=https://ragmir.com PUBLIC_RAGMIR_VERSION=2.4.0 pnpm --filter @jcode.labs/ragmir-landing build
• pnpm public:smoke
• GitNexus detect_changes: low risk, no changed symbols or affected processes