feat: add RPC fallback resilience for unchained outages

[0xApotheosis]

Description

When unchained's MoralisService returns 503 errors, all first-class EVM chains break because their VITE_*_NODE_URL env vars point to unchained's /api/v1/jsonrpc proxy. This PR adds public fallback RPCs at both the viem transport layer and the chain adapter layer so the app degrades gracefully instead of failing completely.

Three layers of change:

1. viemClient.ts — Added FALLBACK_RPC_URLS constants and createFallbackTransport() helper. All 32 EVM chain viem clients now have public fallback RPCs (LlamaRPC, official chain RPCs). Previously only ETH, BSC, and Base had any.

2. EvmBaseAdapter.ts — Added fallback paths for getAccount(), broadcastTransaction(), and getGasFeeData() that route through the viem client (which now has public fallback transports) when unchained fails. Token balances and tx history gracefully degrade to empty during outage.

3. CSP connect-src — Created headers/csps/chains/fallbackRpcUrls.ts as a shared constants file. All 32 chain CSP configs now import and spread fallback RPC origins from this file.

Issue (if applicable)

Risk

Medium — Adds fallback network paths but does not modify any happy-path behavior. When unchained is healthy, no change in behavior. Fallbacks only activate on unchained failure.

Affects all first-class EVM chain adapters (ETH, AVAX, MATIC, ARB, OP, BSC, GNOSIS, BASE) and all second-class EVM chains. No new on-chain transactions or contract interactions introduced — fallback uses the same RPC calls that unchained proxies.

Testing

Engineering

1. To simulate an unchained outage, temporarily set a first-class chain's node URL to a failing endpoint in .env:

   VITE_ETHEREUM_NODE_URL=https://httpstat.us/503
   

2. yarn dev and connect a wallet with ETH
3. Verify on this branch: Native ETH balance loads, console shows Unchained getAccount failed for eip155:1, falling back to direct RPC
4. Verify on develop: Balance fails to load, no fallback
5. Optionally test broadcast and fee estimation with the same broken URL
6. Restore the env var and confirm normal behavior is unchanged

Operations

• 🏁 My feature is behind a flag and doesn't require operations testing (yet)

This is infrastructure resilience — no user-facing UI changes. When unchained is healthy, behavior is identical. When unchained is down, users will see native balances (but not token balances) and can still send transactions, vs a completely broken experience today.

Screenshots (if applicable)

N/A — no UI changes

🤖 Generated with Claude Code

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/rpc-fallback-resilience

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