Disable Lightning across UI surfaces (FeatureFlag.isLightningEnabled = false)

- Add FeatureFlag.isLightningEnabled flag (off)
- Gate dashboard_view_model.hasLightning behind the flag, which transitively
  hides the LightningSwitcher in TopBar and the Lightning Username settings
  row (both already gated on hasLightning)
- Remove CryptoCurrency.btcln from CryptoCurrency.all so it stops appearing
  in any all-currencies pickers/lists

Lightning code paths (cw_bitcoin/lib/lightning/, lightning send/receive
pages) are intentionally left in place so the flag can be flipped back on
once we have an LSP / Greenlight node lined up.

cw_core/lib/crypto_currency.dart

@@ -35,7 +35,7 @@ class CryptoCurrency extends EnumerableItem<int> with Serializable<int> implemen
   static const all = [
     CryptoCurrency.xmr,
     CryptoCurrency.btc,
-    CryptoCurrency.btcln,
+    // CryptoCurrency.btcln,  // Hash Wallet: Lightning disabled, see FeatureFlag.isLightningEnabled
     CryptoCurrency.eth,
     CryptoCurrency.ltc,
     CryptoCurrency.doge,

lib/utils/feature_flag.dart

@@ -7,6 +7,9 @@ class FeatureFlag {
   static const bool isCakePayPurchaseSimulationEnabled = true;
   static const bool isCakePayRedemptionFlowEnabled = false;
   static const bool isExolixEnabled = true;
+  // Hash Wallet: Lightning disabled — running Greenlight/LSP infra is out of
+  // scope for now. Re-enable once we have an LSP relationship lined up.
+  static const bool isLightningEnabled = false;
   static const bool isBackgroundSyncEnabled = true;
   static bool get isInAppTorEnabled => CakeTor.instance is! CakeTorDisabled;
   static const int verificationWordsCount = kDebugMode || kProfileMode ? 0 : 2;

lib/view_model/dashboard/dashboard_view_model.dart

@@ -15,6 +15,7 @@ import 'package:cake_wallet/entities/service_status.dart';
 import 'package:cake_wallet/entities/sync_status_display_mode.dart';
 import 'package:cake_wallet/exchange/exchange_provider_description.dart';
 import 'package:cake_wallet/generated/i18n.dart';
+import 'package:cake_wallet/utils/feature_flag.dart';
 import 'package:cake_wallet/monero/monero.dart';
 import 'package:cake_wallet/nano/nano.dart';
 import 'package:cake_wallet/order/order_provider_description.dart';
@@ -709,7 +710,10 @@ abstract class DashboardViewModelBase with Store {
 
   @computed
   bool get hasLightning =>
-      wallet.type == WalletType.bitcoin && wallet.isSoftwareWallet && bitcoin!.useLightning(wallet);
+      FeatureFlag.isLightningEnabled &&
+      wallet.type == WalletType.bitcoin &&
+      wallet.isSoftwareWallet &&
+      bitcoin!.useLightning(wallet);
 
   @computed
   bool get hasWalletConnect =>