chore: remove unused admin addresses config

[Hunterly01]

Summary

• Remove the unused VITE_ADMIN_ADDRESSES configuration.
• Update the environment-variable documentation to describe the on-chain admin check and clarify that it is a frontend visibility gate, not server-side authorization.

Validation

• pnpm lint — passed
• pnpm typecheck — passed
• pnpm exec turbo run test --concurrency=1 — passed

Closes #1102

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