feat(transaction): add useGasEstimator hook

[lau90eth]

Summary

Every user on Base has overpaid gas at least once. Gas spikes are invisible
until after signing — wallets show "0.00 ETH" and then charge $50.
OnchainKit has zero gas visibility before the user confirms.

This PR adds useGasEstimator, a hook that shows real-time gas fees with
spike detection — before the user clicks "Confirm".

const { estimatedFee, maxFee, isSpike, isSafe, waitTimeMinutes } = 
  useGasEstimator({ maxAcceptableFee: '0.01' });

// "Fee: $0.12 ✅ — safe to send"
// "Gas spike! $4.50 🔴 — wait ~3 min for better rates"

---

What changed

• useGasEstimator hook — estimates real-time gas fees for the current
  Transaction calls. Detects spikes, flags safe windows, and suggests
  wait time when gas is elevated. Auto-refreshes every 15s (configurable).
• estimateGasFee utility — reads onchain gas price via viem
  getGasPrice + estimateGas, maintains a 20-block median history
  for spike detection, calculates worst-case max fee with 150% buffer.
• GasEstimate type — standardized shape for gas data including
  spike/safe flags and wait time estimate.
• Public exports — added to src/transaction/index.ts

---

Usage

import { useGasEstimator } from '@coinbase/onchainkit/transaction';

const { 
  estimatedFee,     // "0.000012" ETH
  maxFee,           // "0.000018" ETH (150% buffer)
  gasUnits,         // bigint
  gasPrice,         // bigint (wei)
  isSpike,          // true if > 2x median network gas
  isSafe,           // true if < 1.2x median
  waitTimeMinutes,  // estimated wait for safe window
  isLoading,
  error,
} = useGasEstimator({
  maxAcceptableFee: '0.01',  // ETH — optional cap
  refreshInterval: 15000,    // ms — default 15s
});

// Usage in UI
{isSpike && (
  <div>⚠️ Gas spike — estimated wait: {waitTimeMinutes} min</div>
)}
{isSafe && (
  <div>✅ Fee: {estimatedFee} ETH — safe to send</div>
)}

---

API

type UseGasEstimatorParams = {
  maxAcceptableFee?: string;  // in ETH, e.g. "0.01"
  refreshInterval?: number;   // ms, default 15000
};

type GasEstimate = {
  estimatedFee: string;       // in ETH
  estimatedFeeUsd?: string;   // if price available
  maxFee: string;             // worst case (150% buffer)
  gasUnits: bigint;
  gasPrice: bigint;
  isSpike: boolean;           // gas > 2x median
  isSafe: boolean;            // gas < 1.2x median
  waitTimeMinutes?: number;   // estimated wait for safe window
};

---

Notes to reviewers

• Zero external API dependency — reads directly from chain via viem
  getGasPrice and estimateGas. Works on any chain configured in
  OnchainKitProvider.
• Spike detection — maintains a 20-block in-memory median history
  per chain. isSpike = current gas > 2x median. isSafe = current
  gas < 1.2x median.
• waitTimeMinutes — heuristic based on historical gas volatility
  in the cached 20-block window. Not guaranteed — presented as an
  estimate to guide user decisions.
• maxAcceptableFee — optional. When set, isSafe also requires
  estimated fee to be below this threshold.
• Auto-refreshes every refreshInterval ms. Clears on unmount.
• Reads calls from useTransactionContext automatically.

---

Testing

• useGasEstimator.test.ts — 4 tests
• estimateGas.test.ts — utility unit tests
• All 389 test files pass (2697 tests)

Test cases covered:

• Empty state when no calls present → returns null
• Normal gas estimate → correct fee calculation
• Spike detection → isSpike: true when gas > 2x median
• API/onchain error → graceful fallback, error state exposed

---

Risk

Low. Read-only hook. Uses getGasPrice and estimateGas —
both are standard viem read operations with zero side effects.

Returns null when no calls are present.
Does not modify TransactionProvider, TransactionButton,
or any existing transaction logic.

[vercel]

@lau90eth is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[cb-heimdall]

🟡 Heimdall Review Status

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