Expose server helpers via @solana/client/server

[GuiBibeau]

Summary

• keep server-only helpers inside @solana/client/server (cluster monikers + signer load/save), no separate server-client package
• clean export map to expose types at the root and server subpath without browser bloat
• add tests for cluster resolution and server signer ergonomics; ensure builds/lint/typecheck pass with the new layout

Testing

• pnpm --filter @solana/client test -- server/signers.test.ts utils/cluster.test.ts
• pnpm --filter @solana/client build