fix: use black text on green buttons for consistency (#473)

All green (mostroGreen/activeColor) buttons in NWC widgets and wallet
screens were using white foreground text. The rest of the app uses
black text on green buttons. Updated 5 instances across 4 files:
- nwc_payment_widget.dart (1)
- nwc_invoice_widget.dart (2)
- wallet_settings_screen.dart (1)
- connect_wallet_screen.dart (1)

Co-authored-by: mostronator <mostronator@users.noreply.github.com>

Author: Mostronator

lib/features/wallet/screens/connect_wallet_screen.dart

@@ -200,7 +200,7 @@ class _ConnectWalletScreenState extends ConsumerState<ConnectWalletScreen> {
                         ),
                         style: ElevatedButton.styleFrom(
                           backgroundColor: AppTheme.activeColor,
-                          foregroundColor: Colors.white,
+                          foregroundColor: Colors.black,
                           padding: const EdgeInsets.symmetric(vertical: 14),
                           shape: RoundedRectangleBorder(
                             borderRadius: BorderRadius.circular(8),

lib/features/wallet/screens/wallet_settings_screen.dart

@@ -291,7 +291,7 @@ class WalletSettingsScreen extends ConsumerWidget {
                     label: Text(S.of(context)!.connectWallet),
                     style: ElevatedButton.styleFrom(
                       backgroundColor: AppTheme.activeColor,
-                      foregroundColor: Colors.white,
+                      foregroundColor: Colors.black,
                       padding: const EdgeInsets.symmetric(vertical: 14),
                       shape: RoundedRectangleBorder(
                         borderRadius: BorderRadius.circular(8),

lib/shared/widgets/nwc_invoice_widget.dart

@@ -172,7 +172,7 @@ class _NwcInvoiceWidgetState extends ConsumerState<NwcInvoiceWidget> {
             ),
             style: ElevatedButton.styleFrom(
               backgroundColor: AppTheme.mostroGreen,
-              foregroundColor: Colors.white,
+              foregroundColor: Colors.black,
               shape: RoundedRectangleBorder(
                 borderRadius: BorderRadius.circular(12),
               ),
@@ -283,7 +283,7 @@ class _NwcInvoiceWidgetState extends ConsumerState<NwcInvoiceWidget> {
                   : null,
               style: ElevatedButton.styleFrom(
                 backgroundColor: AppTheme.mostroGreen,
-                foregroundColor: Colors.white,
+                foregroundColor: Colors.black,
                 shape: RoundedRectangleBorder(
                   borderRadius: BorderRadius.circular(12),
                 ),

lib/shared/widgets/nwc_payment_widget.dart

@@ -171,7 +171,7 @@ class _NwcPaymentWidgetState extends ConsumerState<NwcPaymentWidget> {
             ),
             style: ElevatedButton.styleFrom(
               backgroundColor: AppTheme.mostroGreen,
-              foregroundColor: Colors.white,
+              foregroundColor: Colors.black,
               disabledBackgroundColor: Colors.grey.shade700,
               shape: RoundedRectangleBorder(
                 borderRadius: BorderRadius.circular(12),