Forward the registry routes from the deployed origin

[lmcorbalan]

User story / Problem statement

api/rpc.ts forwards a wallet-service method the app no longer calls, and a deployed https page cannot reach a plain-http registry.

Expected outcome

The deployed build reaches the registry through the app's own function, which forwards the three routes the dApp uses and refuses everything else.

Acceptance criteria

• api/registry/[...path].ts forwards the two metadata routes and the transfer-factory route
• Any other path answers 403
• A missing REGISTRY_URL answers 500 naming the variable
• api/rpc.ts is deleted
• knip.json's frontend entry matches the nested function

Technical notes

knip's entry is api/*.ts and will not match a file one directory down, so the new function would read as unused and fail the gate. dapp/frontend/CLAUDE.md's one-module-per-route rule needs the catch-all recorded. Hosting a registry for the deployed demo stays unsolved and out of scope.