fix(frontend): fail over between RPC endpoints and surface event load errors

[masusanou]

Summary

• Resolve read-only RPC endpoints in priority order: NEXT_PUBLIC_PROVIDER_RPC, comma-separated NEXT_PUBLIC_PROVIDER_RPC_FALLBACKS, then the RPC derived from the existing thirdweb client ID.
• Validate every configured endpoint against NEXT_PUBLIC_CHAIN_ID, then retry ethers-based reads only for transport, authentication, rate-limit, and recognized endpoint-availability failures.
• Preserve semantic JSON-RPC errors such as a genuine contract revert instead of replacing them with fallback data.
• Build activeChain.rpc without placing an undefined environment value first.
• Propagate both event-record and record-count errors from useEvents, ignore stale async completions, and show a localized error on /events instead of rendering an indistinguishable empty list.

Incident context

The production Polygon RPC currently responds with HTTP 403 and App is inactive. Reading the deployed EventManager through another Polygon RPC returns getEventRecordCount() = 1070, so the on-chain data remains intact and the failure is in the read path.

Important scope limitation

The /events reads themselves go through thirdweb SDK v3. That SDK selects the first entry in activeChain.rpc; it does not perform runtime failover across the array. Therefore this PR makes the outage visible on /events, provides runtime failover for the ethers-based reads in contractMethods.ts, and allows thirdweb to use the derived RPC when NEXT_PUBLIC_PROVIDER_RPC is missing. It does not make thirdweb /events reads fail over when a configured first endpoint is unhealthy.

Restoring production still requires replacing/reactivating NEXT_PUBLIC_PROVIDER_RPC and rebuilding/redeploying the frontend.

Validation

• yarn tsc --noEmit — pass
• yarn next lint --dir src — pass, no warnings or errors
• yarn build — pass; optimized production build compiled and generated all 33 static pages (the env-less verification worktree emitted expected thirdweb clientId warnings)
• Prettier check for the changed formatted TypeScript files — pass
  • contractMethods.ts is already Prettier-noncompliant on staging; this PR deliberately avoids unrelated whole-file formatting
• git diff --check HEAD~1 HEAD — pass
• Runtime smoke/regression tests using the actual rpcProviders.ts:
  • local primary endpoint returns HTTP 403 plus a JSON-RPC error; provider retries 137.rpc.thirdweb.com; deployed EventManager getEventRecordCount() returns 1070
  • an endpoint reporting chain ID 1 is rejected when NEXT_PUBLIC_CHAIN_ID=137
  • a semantic execution reverted response is preserved as CALL_EXCEPTION, with zero calls to the fallback endpoint
  • a failed sole endpoint contract call throws instead of returning an empty value
  • no configured endpoint throws No RPC endpoint is configured. Set NEXT_PUBLIC_PROVIDER_RPC.
• Added-line security scan — no findings

The /events error state has not been manually verified in a browser.

Failover semantics

Chain checks are cached only after successful validation. Availability failures move to the next endpoint, while semantic RPC failures are returned immediately to the caller.

Closes #556

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