Add fee validation for sweepAll transactions to prevent excessive fees

[Copilot]

When sweeping all UTXOs, users with many small UTXOs at high fee rates could pay fees exceeding their balance. This adds validation to reject sweeps where fees exceed 20% of total balance.

Changes

• Fee percentage check: Validates fee / totalInput < 20% before proceeding with sweepAll transactions
• Error messaging: Clear error with actual fee percentage and actionable suggestion to consolidate UTXOs at lower fee rates
• Constant: Defined MAX_SWEEP_FEE_PERCENTAGE = 20 at module level for maintainability

Example

// Before: Would attempt transaction even with 50% fee
createSendTransaction(utxos, address, 0, 100, ..., { sweepAll: true })

// After: Throws clear error when fee exceeds threshold
// Error: Fee too high: 25000 sats (50.0%) exceeds 20% of total balance (50000 sats).
// This may happen when sweeping many small UTXOs with high fee rates.
// Consider consolidating UTXOs at a lower fee rate first.

Validation occurs after fee calculation but before insufficient funds check, ensuring fee percentage errors are surfaced first when both conditions fail.

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