fix: auth token handling, add tests

- Read bearer token from set-auth-token header
- Add mounted checks to prevent setState after dispose
- Add mocktail for testing
- Add widget tests for login, clients, events screens
- Add unit tests for auth provider, API client
- 110 tests passing

lib/shared/services/api_client.dart

@@ -51,6 +51,13 @@ class ApiClient {
       'password': password,
       'name': name,
     });
+    
+    // Store session token from header (BetterAuth auto-signs in after signup)
+    final authToken = response.headers.value('set-auth-token');
+    if (authToken != null) {
+      await _storage.write(key: 'session_token', value: authToken);
+    }
+    
     return response.data;
   }
   
@@ -63,9 +70,10 @@ class ApiClient {
       'password': password,
     });
     
-    // Store session token
-    if (response.data['token'] != null) {
-      await _storage.write(key: 'session_token', value: response.data['token']);
+    // Store session token from header (BetterAuth bearer plugin)
+    final authToken = response.headers.value('set-auth-token');
+    if (authToken != null) {
+      await _storage.write(key: 'session_token', value: authToken);
     }
     
     return response.data;