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/pubspec.yaml

@@ -0,0 +1,13 @@
+name: sxchnge_auth
+description: Shared authentication providers and service contracts for SXCHNGE Flutter clients.
+version: 0.1.0
+publish_to: none
+environment:
+  sdk: '>=3.13.0 <4.0.0'
+  flutter: '>=3.47.0'
+dependencies:
+  flutter:
+    sdk: flutter
+dev_dependencies:
+  flutter_test:
+    sdk: flutter