feat(wallet): flip to Cardano mainnet

- CardanoNetworkConfig.NETWORK = MAINNET
- Koios API: api.koios.rest (was preprod.koios.rest)
- Explorer: cardanoscan.io (was preprod.cardanoscan.io)
- Address prefix: addr1 (was addr_test1)
- WalletPanelNode: use config for explorer URL

To flip back to testnet, change one line:
  val NETWORK = CardanoNetwork.TESTNET

features/wallet/impl/src/main/kotlin/io/element/android/features/wallet/impl/cardano/CardanoNetworkConfig.kt

@@ -17,10 +17,10 @@ enum class CardanoNetwork {
 /**
  * Centralized network configuration for the Cardano wallet.
  *
- * To switch networks, change [NETWORK] to [CardanoNetwork.MAINNET].
+ * To switch networks, change [NETWORK] to [CardanoNetwork.TESTNET].
  * All derived values (network ID, API URLs) will update automatically.
  *
- * **Current configuration: TESTNET (preprod)**
+ * **Current configuration: MAINNET**
  */
 object CardanoNetworkConfig {
     /**
@@ -29,7 +29,7 @@ object CardanoNetworkConfig {
      * Set to [CardanoNetwork.TESTNET] for development/testing.
      * Set to [CardanoNetwork.MAINNET] for production.
      */
-    val NETWORK: CardanoNetwork = CardanoNetwork.TESTNET
+    val NETWORK: CardanoNetwork = CardanoNetwork.MAINNET
 
     /**
      * Cardano network ID.

features/wallet/impl/src/main/kotlin/io/element/android/features/wallet/impl/panel/WalletPanelNode.kt

@@ -55,11 +55,7 @@ class WalletPanelNode @AssistedInject constructor(
                 eventSink = { event ->
                     when (event) {
                         is WalletPanelEvent.OpenTransaction -> {
-                            val url = if (CardanoNetworkConfig.NETWORK_NAME != "mainnet") {
-                                "https://preprod.cardanoscan.io/transaction/${event.txHash}"
-                            } else {
-                                "https://cardanoscan.io/transaction/${event.txHash}"
-                            }
+                            val url = "${CardanoNetworkConfig.EXPLORER_BASE_URL}/transaction/${event.txHash}"
                             val intent = Intent(Intent.ACTION_VIEW, Uri.parse(url))
                             context.startActivity(intent)
                         }