[#205] Shared RPC client with fallback transport

[realproject7]

Summary

• Create lib/rpc.ts with shared public client for Base/Base Sepolia
• Chain selected via NEXT_PUBLIC_CHAIN_ID (default: 84532 / Base Sepolia)
• Fallback transport: tries NEXT_PUBLIC_RPC_URL first, falls back to chain default
• Update lib/viem.ts to re-export from lib/rpc.ts for backward compatibility
• Aligned with wagmi config (same env vars, same chain selection)

Test plan

• tsc --noEmit passes
• eslint passes
• Existing imports from lib/viem still work
• New code can import from lib/rpc directly

Fixes #205

🤖 Generated with Claude Code

[project7-interns]

T2b Review: APPROVED

Clean RPC client setup. Findings:

1. Chain selection — NEXT_PUBLIC_CHAIN_ID with default 84532 (Base Sepolia). Consistent with wagmi config's approach from PR #58.
2. Fallback transport — custom RPC with fallback to default public RPC when NEXT_PUBLIC_RPC_URL is set; plain http() otherwise. Good resilience pattern.
3. Backward compat — lib/viem.ts re-exports from lib/rpc.ts so existing imports don't break. Clean migration path.
4. Singleton — module-level client instantiation ensures a single shared instance.
5. CI green.

No issues found.

[project7-interns]

Verdict: APPROVE

Summary

This PR adds the shared RPC client foundation requested for P3-5 and keeps existing lib/viem imports working through a clean compatibility re-export. The repository check is passing.

Findings

• None.

Decision

Approving because the new lib/rpc.ts aligns with the existing chain-selection env vars used by wagmi, preserves backward compatibility for current callers, and stays narrowly scoped to the requested RPC client foundation.