fix: make stateless members static and internalize app-only types (CA1822, CA1515)

[DeepDiver1975]

• CA1822 (10): AboutViewModel's five display properties are bound from AboutView via {Binding} and must stay instance members — suppressed with justification; IncidentRepository Save/Load/TryReadState and MasterDataStore GetOrCreate/Save were stateless and are now static, with callers (IncidentStore, MasterDataProvider, test suites) updated.
• CA1515 (7): the four desktop App types (AppPaths, IncidentHostController, StorageProviderFileDialogService, SystemAlarmService) are only consumed inside the App assembly plus LageBuch.App.Tests — internalized and granted InternalsVisibleTo for the test project. The test helpers TestAppBuilder, PdfAssert and Domain.Tests FixedClock are internal; xUnit still discovers all tests (801 passed, 1 skipped — unchanged).
• Verification: dotnet build (AnalysisMode=All) shows CA1822 = 0, CA1515 = 0, zero errors, no other rule delta from main; full test suite green.