fix(onchain): surface RPC fetch failures instead of silently returning empty

[0xblckmrq]

Summary

• getAttestationData was swallowing all RPC errors and returning undefined, making it impossible to distinguish a genuine empty attestation from a fetch failure
• onChainContext then silently reset activeChainProviders to [] — no error state, no retry affordance
• ~4k Datadog events/week, ~297 distinct sessions affected (WalletConnect relay + Alchemy RPC errors)
• Fixes holonym-foundation/internal-docs#2543

Changes

File	Change
app/utils/onChainStamps.ts	getAttestationData re-throws after logging — return type narrows from AttestationData | undefined to AttestationData
app/context/onChainContext.tsx	Per-chain try/catch around getAttestationData; sets onChainFetchFailed=true when connected chain fails; exposed on OnChainContextState
app/components/DashboardValidStampsPanel.tsx	Shows "Couldn't load on-chain data / Try again" when onChainFetchFailed is true
app/__test-fixtures__/contextTestHelpers.tsx	Added onChainFetchFailed: false to test fixture

Test plan

• Happy path: on-chain stamps load normally — onChainFetchFailed stays false, no error UI shown
• RPC failure: simulate by setting a bad rpcUrl in chains config — confirm error banner appears with "Try again" button
• Retry: click "Try again" — confirm readOnChainData re-fires and clears the error on success
• Non-connected chains failing silently: confirm only the connected chain's failure sets onChainFetchFailed

cc @calebtuttle

🤖 Generated with Claude Code

[0xblckmrq]

Closing — this PR targets the holonym-foundation fork which uses a removed onChainContext architecture. The correct target is passportxyz/passport. A new PR will be filed there against the useQueries implementation. See holonym-foundation/internal-docs#2543.