feat: (WIP) add nfc_manager plugin and non-working UI elements

added Android permissions, but
TODO add iOS entitlements etc. ... see https://github.com/okadan/flutter-nfc-manager?tab=readme-ov-file#setup

lib/pages/wallet_view/multisig_setup_view/multisig_coordinator.dart

@@ -0,0 +1,216 @@
+import 'package:bip48/bip48.dart';
+import 'package:coinlib/coinlib.dart';
+
+/// Represents the parameters needed to create a shared multisig account.
+class MultisigParams {
+  /// Number of required signatures (M in M-of-N).
+  final int threshold;
+
+  /// Total number of participants (N in M-of-N).
+  final int totalCosigners;
+
+  /// BIP44 coin type (e.g., 0 for Bitcoin mainnet).
+  final int coinType;
+
+  /// BIP44/48 account index.
+  final int account;
+
+  /// BIP48 script type (e.g., p2sh, p2wsh).
+  final Bip48ScriptType scriptType;
+
+  /// Creates a new set of multisig parameters.
+  const MultisigParams({
+    required this.threshold,
+    required this.totalCosigners,
+    required this.coinType,
+    required this.account,
+    required this.scriptType,
+  });
+
+  /// Validates the parameters for consistency.
+  ///
+  /// Returns true if all parameters are valid:
+  /// - threshold > 0
+  /// - threshold <= totalCosigners
+  /// - account >= 0
+  /// - coinType >= 0
+  bool isValid() {
+    return threshold > 0 &&
+        threshold <= totalCosigners &&
+        account >= 0 &&
+        coinType >= 0;
+  }
+}
+
+/// Represents a participant in the multisig setup process.
+class CosignerInfo {
+  /// The cosigner's BIP48 account-level extended public key.
+  final String accountXpub;
+
+  /// Position in the sorted set of cosigners (0-based).
+  final int index;
+
+  /// Creates info about a cosigner participant.
+  const CosignerInfo({
+    required this.accountXpub,
+    required this.index,
+  });
+}
+
+/// Coordinates the creation of a shared multisig account between multiple users.
+class MultisigCoordinator {
+  /// Local master key if available (otherwise uses accountXpub).
+  final HDPrivateKey? localMasterKey;
+
+  /// Parameters for the shared multisig wallet.
+  final MultisigParams params;
+
+  /// Collected cosigner information.
+  final List<CosignerInfo> _cosigners = [];
+
+  /// Local account xpub when not using master key.
+  String? _accountXpub;
+
+  /// Creates a coordinator using the local HD master private key.
+  ///
+  /// Uses the provided [localMasterKey] to derive the account xpub that will
+  /// be shared with other cosigners.
+  MultisigCoordinator({
+    required this.localMasterKey,
+    required this.params,
+  }) {
+    if (!params.isValid()) {
+      throw ArgumentError('Invalid multisig parameters');
+    }
+  }
+
+  /// Creates a coordinator using a pre-derived account xpub.
+  ///
+  /// This constructor should be used when you only want to verify addresses
+  /// or don't have access to the master private key.
+  MultisigCoordinator.fromXpub({
+    required String accountXpub,
+    required this.params,
+  }) : localMasterKey = null {
+    if (!params.isValid()) {
+      throw ArgumentError('Invalid multisig parameters');
+    }
+    _accountXpub = accountXpub;
+  }
+
+  /// Gets this user's account xpub that needs to be shared with other cosigners.
+  ///
+  /// If created with a master key, derives the account xpub at the BIP48 path.
+  /// If created with fromXpub, returns the provided account xpub.
+  String getLocalAccountXpub() {
+    if (_accountXpub != null) {
+      return _accountXpub!;
+    }
+
+    if (localMasterKey == null) {
+      throw StateError('No master key or account xpub available');
+    }
+
+    final path = bip48DerivationPath(
+      coinType: params.coinType,
+      account: params.account,
+      scriptType: params.scriptType,
+    );
+    final accountKey = localMasterKey!.derivePath(path);
+    return accountKey.hdPublicKey.encode(bitcoinNetwork.mainnet.pubHDPrefix);
+  }
+
+  /// Adds a cosigner's account xpub to the set.
+  ///
+  /// Throws [StateError] if all cosigners have already been added.
+  void addCosigner(String accountXpub) {
+    if (_cosigners.length >= params.totalCosigners - 1) {
+      throw StateError('All cosigners have been added');
+    }
+
+    // Assign index based on current position
+    _cosigners.add(CosignerInfo(
+      accountXpub: accountXpub,
+      index: _cosigners.length + 1, // Local user is always index 0.
+    ));
+  }
+
+  /// Checks if all required cosigner information has been collected.
+  bool isComplete() {
+    return _cosigners.length == params.totalCosigners - 1;
+  }
+
+  /// Creates the local wallet instance once all cosigners are added.
+  ///
+  /// Throws [StateError] if not all cosigners have been added yet.
+  Bip48Wallet createWallet() {
+    if (!isComplete()) {
+      throw StateError('Not all cosigners have been added');
+    }
+
+    // Create wallet with either our master key or xpub
+    final wallet = localMasterKey != null
+        ? Bip48Wallet(
+            masterKey: localMasterKey,
+            coinType: params.coinType,
+            account: params.account,
+            scriptType: params.scriptType,
+            threshold: params.threshold,
+            totalKeys: params.totalCosigners,
+          )
+        : Bip48Wallet(
+            accountXpub: _accountXpub,
+            coinType: params.coinType,
+            account: params.account,
+            scriptType: params.scriptType,
+            threshold: params.threshold,
+            totalKeys: params.totalCosigners,
+          );
+
+    // Add all cosigner xpubs.
+    for (final cosigner in _cosigners) {
+      wallet.addCosignerXpub(cosigner.accountXpub);
+    }
+
+    return wallet;
+  }
+
+  /// Verifies that derived addresses match between all participants.
+  ///
+  /// Takes a list of [sharedAddresses] that other participants derived, along
+  /// with the [indices] used to derive them and whether they are [isChange]
+  /// addresses.
+  ///
+  /// Returns true if all provided addresses match our local derivation.
+  bool verifyAddresses(List<String> sharedAddresses,
+      {required List<int> indices, required bool isChange}) {
+    if (!isComplete()) return false;
+
+    final wallet = createWallet();
+    for (final idx in indices) {
+      final derivedAddress =
+          wallet.deriveMultisigAddress(idx, isChange: isChange);
+      final sharedAddress = sharedAddresses[indices.indexOf(idx)];
+      if (derivedAddress != sharedAddress) return false;
+    }
+    return true;
+  }
+
+  /// Gets a list of derived addresses for verification.
+  ///
+  /// Derives addresses at the specified [indices] on either the external
+  /// or change chain based on [isChange].
+  ///
+  /// Throws [StateError] if not all cosigners have been added yet.
+  List<String> getVerificationAddresses(
+      {required List<int> indices, required bool isChange}) {
+    if (!isComplete()) {
+      throw StateError('Not all cosigners have been added');
+    }
+
+    final wallet = createWallet();
+    return indices
+        .map((idx) => wallet.deriveMultisigAddress(idx, isChange: isChange))
+        .toList();
+  }
+}