Enable de-DE culture with ICU runtime image

Elternbeirat.Web/Elternbeirat.Web.csproj

@@ -5,6 +5,10 @@
         <Nullable>enable</Nullable>
         <ImplicitUsings>enable</ImplicitUsings>
         <BlazorDisableThrowNavigationException>true</BlazorDisableThrowNavigationException>
+        <!-- The pages format dates in German (de-DE). Without this the runtime
+             falls back to invariant-only globalization and any named culture
+             throws. Needs an ICU-carrying runtime image (see Dockerfile). -->
+        <InvariantGlobalization>false</InvariantGlobalization>
     </PropertyGroup>
 
     <!-- The test project uses WebApplicationFactory<Program>, which needs the