add signet support

src/main/java/com/sparrowwallet/sparrow/MainApp.java

@@ -142,6 +142,11 @@ public class MainApp extends Application {
             Network.set(Network.TESTNET);
         }
 
+        File signetFlag = new File(Storage.getSparrowHome(), "network-" + Network.SIGNET.getName());
+        if(signetFlag.exists()) {
+            Network.set(Network.SIGNET);
+        }
+
         if(Network.get() != Network.MAINNET) {
             getLogger().info("Using " + Network.get() + " configuration");
         }

src/main/java/com/sparrowwallet/sparrow/net/ElectrumServer.java

@@ -632,6 +632,9 @@ public class ElectrumServer {
             Map<Integer, Double> targetBlocksFeeRatesSats = new TreeMap<>();
             for(Integer target : targetBlocksFeeRatesBtcKb.keySet()) {
                 long minFeeRateSatsKb = (long)(targetBlocksFeeRatesBtcKb.get(target) * Transaction.SATOSHIS_PER_BITCOIN);
+                if(minFeeRateSatsKb < 0) {
+                    minFeeRateSatsKb = 1000;
+                }
                 targetBlocksFeeRatesSats.put(target, minFeeRateSatsKb / 1000d);
             }