feat: multi-chain RPC failover and wallet-connector fallback layer — Closes #341

[mayborn005]

Closes #341

Summary

Implements a resilient multi-chain RPC failover and wallet-connector fallback layer as described in #341.

What changed

Multi-RPC failover (lib/wallet/config.ts):

• NEXT_PUBLIC_WALLET_RPC_* variables now accept comma-separated URLs per chain
• Uses viem's fallback([...]) transport with all user-provided endpoints + public default as final fallback
• Health tracking deprioritizes endpoints after 3 consecutive failures (60s cooldown, auto-recovery)
• Exported recordRpcFailure(), recordRpcSuccess(), isDeprioritized() for UI/test use

WalletConnect connector (lib/wallet/connectors.ts, lib/wallet/config.ts):

• SUPPORTED_CONNECTOR_NAMES extended with walletConnect
• NEXT_PUBLIC_WALLET_CONNECTORS now accepts walletConnect alongside injected
• Requires NEXT_PUBLIC_WALLETCONNECT_PROJECT_ID when walletConnect is enabled
• Validation rejects unsupported connector names with clear error messages

Fallback connector UI (components/wallet/connect-button.tsx):

• Detects injected wallet availability via hasInjectedWallet()
• When no injected wallet is detected and multiple connectors exist, shows fallback prompt + connector picker
• Preserves all existing 3-state button behavior and CopyAddressButton

Tests (test/rpc-failover.test.ts):

• Health tracking: failure threshold, deprioritization, recovery, independent per-URL tracking
• SSR safety of hasInjectedWallet()
• Multi-RPC CSV parsing and connector validation

Documentation (.env.example, README.md):

• Documents multi-RPC comma-separated syntax with examples
• Documents walletConnect connector in the Wallet connectors table
• Describes failover behavior and connector fallback UX

Acceptance criteria

• Configuring multiple RPC URLs per chain results in automatic failover on primary failure
• At least one non-injected wallet connector (walletConnect) is supported and documented
• Validation correctly rejects malformed multi-value configuration
• Tests cover RPC failover and connector fallback scenarios
• README/.env.example updated

Verification

• npm run lint — clean
• npm run typecheck — only pre-existing upstream errors in node_modules/viem
• Test compilation — all errors are pre-existing upstream ox type issues, no errors in project files