Add shared authentication and email verification services

Provide reusable models, injectable service contracts, lifecycle-safe providers and explicit dummy implementations for login, signup and verification. Add focused error and async lifecycle tests. Related to #4 and #5.

frontend/packages/sxchnge_auth/lib/src/models/verification.dart

@@ -0,0 +1,31 @@
+enum VerificationStatus {
+  idle,
+  sending,
+  sent,
+  checking,
+  verified,
+  expired,
+  invalid,
+  error,
+}
+
+enum VerificationError { expired, invalid, internal }
+
+class VerificationException implements Exception {
+  const VerificationException(this.code);
+  final VerificationError code;
+}
+
+class VerificationResult {
+  const VerificationResult({required this.isSimulated});
+  final bool isSimulated;
+}
+
+class VerificationState {
+  const VerificationState(this.status, {this.isSimulated = false});
+  final VerificationStatus status;
+  final bool isSimulated;
+  bool get isLoading =>
+      status == VerificationStatus.sending ||
+      status == VerificationStatus.checking;
+}