fix: resolve solana rent exempt simulation error when sweeping native full balances

lib/crypto-engine.ts

@@ -264,9 +264,11 @@ export class CryptoEngine {
             const destPubKey = new PublicKey(destinationAddress);
             
             if (tokenSymbol === 'SOL' || tokenSymbol === 'NATIVE') {
-                // Get balance and send almost all (leave 5000 lamports for rent)
+                // Get balance and send almost all (leave exactly 0 to avoid rent-exempt errors)
+                // A standard Solana signature costs exactly 5000 lamports
                 const balance = await this.solConnection.getBalance(senderKeypair.publicKey);
-                const sendAmount = balance - 10000; // leave 0.00001 SOL for fees
+                const fee = 5000;
+                const sendAmount = balance - fee; 
                 
                 if (sendAmount <= 0) {
                     return { success: false, txHash: null, error: 'Insufficient SOL balance' };