fix(docs): add rate-limit warning for publicnode fallback RPC in buil…

[schoolkamsergj]

Closes #1356

What changed? Why?
The build-app guide mentioned https://base-sepolia-rpc.publicnode.com as a fallback RPC endpoint but did not warn that it is also rate-limited and unsuitable for production — unlike the main endpoint which already had this warning.

Changes
Updated the Note in Step 4 to explicitly state that the publicnode fallback is rate-limited and for testing only, consistent with the existing warning for the primary endpoint.

[cb-heimdall]

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