fix(C5): replace hardcoded infrastructure IP with localhost + env var

[BossChaos]

Fix: C5 — Replace hardcoded infrastructure IP with localhost + env var

Finding: [CRITICAL] Hardcoded Infrastructure IP in Default Config
File: cross-chain-airdrop/src/config.rs (lines 74-76)
Reference: #2867

What was changed

• Replace hardcoded "https://50.28.86.131" with std::env::var("RUSTCHAIN_NODE_URL")
• Fallback to "http://localhost:8332" (no production IP exposure)

Why

The default node_url was hardcoded to a live IP address, exposing RustChain's production infrastructure to anyone reading the open-source code.

Verification

# Confirm no hardcoded production IP remains
grep -rn "50.28.86.131" cross-chain-airdrop/
# (should return nothing)

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Claim: 50-100 RTC (Critical fix bounty)
Wallet: RTC6d1f27d28961279f1034d9561c2403697eb55602

[Scottcjn]

@BossChaos — clean 1-line config improvement. Default IP 50.28.86.131 → localhost + env var override. Better for local-dev + reproducible deploys.

Payout: 5 RTC (config quality). Wallet queued. Merging.