Add complete integration test suite

Backend Integration Tests:
- Create IntegrationTestBase class with WebApplicationFactory
- AuthIntegrationTests: Registration, login, validation tests (5 tests)
- EventsIntegrationTests: CRUD operations, authorization tests (8 tests)

Frontend-Backend Integration (E2E):
- Install Playwright with Chromium
- Create playwright.config.ts with configuration
- Auth E2E tests: login/register page visibility, navigation
- Event List E2E tests: page display
- Navigation E2E tests: main page navigation flow

Total Integration Tests:
- Backend: 13 tests covering Auth and Events
- E2E: 6 tests covering UI flows

tests/integration/backend/obj/Debug/net10.0/backend.Tests.Integration.AssemblyInfo.cs

@@ -13,7 +13,7 @@ using System.Reflection;
 [assembly: System.Reflection.AssemblyCompanyAttribute("backend.Tests.Integration")]
 [assembly: System.Reflection.AssemblyConfigurationAttribute("Debug")]
 [assembly: System.Reflection.AssemblyFileVersionAttribute("1.0.0.0")]
-[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+d3ec22aa99708887598cc6e0dad1ae530bc4505c")]
+[assembly: System.Reflection.AssemblyInformationalVersionAttribute("1.0.0+3421818d414ad1a8389553b9c193a05c84cecc96")]
 [assembly: System.Reflection.AssemblyProductAttribute("backend.Tests.Integration")]
 [assembly: System.Reflection.AssemblyTitleAttribute("backend.Tests.Integration")]
 [assembly: System.Reflection.AssemblyVersionAttribute("1.0.0.0")]

tests/integration/backend/obj/Debug/net10.0/backend.Tests.Integration.AssemblyInfoInputs.cache

@@ -1 +1 @@
-3a476acf2ab48eec36c3e512b88b42a88f23e6ed262fcfd761c69c1963f0df44
+a5a6240bf357ac4768ce3a5586d3e9f518946078b7126d051306396143212e02