fix(config): remove dead Thesis/Staked Electrum URLs

Replace defunct Thesis and Staked Electrum servers with BOAR endpoints:
- Mainnet: remove 3 dead URLs, keep single BOAR server
- Testnet: swap dead Thesis URL for BOAR testnet server
- Update test fixtures to match new URL configuration

Author: lrsaturnino

config/_electrum_urls/mainnet

@@ -1,4 +1 @@
-wss://electrumx-server.tbtc.network:8443
 wss://electrum.boar.network:2083
-wss://bitcoin.threshold.p2p.org:50004
-wss://electrumx.prod-utility-eks-us-west-2.staked.cloud:443

config/_electrum_urls/testnet

@@ -1 +1 @@
-wss://electrumx-server.test.tbtc.network:8443
+wss://electrum.testnet.boar.network:443/QxbJgaSLUHqrgAa9BW7bDpnGPxrlhnCa

config/electrum_test.go

@@ -18,26 +18,18 @@ func TestResolveElectrum(t *testing.T) {
 	}{
 		bitcoin.Mainnet: {
 			expectedConfig: []electrum.Config{
-				{
-					URL: "wss://electrumx-server.tbtc.network:8443",
-				},
 				{
 					URL: "wss://electrum.boar.network:2083",
 				},
-				{
-					URL: "wss://bitcoin.threshold.p2p.org:50004",
-				},
-				{
-					URL: "wss://electrumx.prod-utility-eks-us-west-2.staked.cloud:443",
-				},
 			},
 		},
 		bitcoin.Testnet: {
 			expectedConfig: []electrum.Config{
 				{
-					URL: "wss://electrumx-server.test.tbtc.network:8443",
+					URL: "wss://electrum.testnet.boar.network:443/QxbJgaSLUHqrgAa9BW7bDpnGPxrlhnCa",
 				},
-			}},
+			},
+		},
 		bitcoin.Regtest: {
 			expectedConfig: []electrum.Config{
 				{