Revamp frontend product-sdk integration and registry UI

[charlesHetterich]

Summary

• migrate the frontend onto product-sdk chain, contracts, descriptors, Bulletin, and host helpers
• route registry data through the product-sdk/CDM flow and expose registry resolution from @dotdm/env
• redesign the registry homepage around the new black Contract Registry layout with reusable search and install-command components
• add skeleton/loading polish and simplify the network picker/header chrome
• update top-bar links so Docs and Github point to the CDM repo and Playground opens https://playground.dot via the Desktop host-aware external-link handler
• remove obsolete frontend-only assets/components including the grain canvas, starter SVGs, and footer

Verification

• pnpm --filter @dotdm/env build
• pnpm --filter @dotdm/frontend lint
• pnpm --filter @dotdm/frontend build

Notes

• Product-sdk descriptors still produce descriptor chunks, matching the current playground-app integration pattern.
• External links use @parity/product-sdk-host so Polkadot Desktop can handle navigation, with browser fallback outside the host.
• Host-only product-sdk connections now fail with explicit timeout/container errors instead of sitting indefinitely on the connecting state.