docs: add culture management migration warnings for Server/Auto

Prepare for Blazor Server/Auto migration by documenting current
WASM-specific culture initialization approach and providing
detailed migration checklist in COPILOT_CONTEXT.md.

EnvelopeGenerator.ReceiverUI/Program.cs

@@ -50,7 +50,25 @@ ReportStorageWebExtension.RegisterExtensionGlobal(new InMemoryReportStorageWebEx
 
 var host = builder.Build();
 
-// Initialize culture BEFORE running the app
+// ⚠️ IMPORTANT: BLAZOR WASM-SPECIFIC CULTURE INITIALIZATION
+// This approach sets DefaultThreadCurrentCulture globally, which is SAFE for WebAssembly
+// because each user runs their own isolated app instance in their browser.
+// 
+// ⚠️ TODO: REMOVE/REFACTOR WHEN MIGRATING TO BLAZOR SERVER/AUTO
+// In Server/Auto render modes, this is DANGEROUS because:
+// - Server runs a single shared instance for all users
+// - Setting global culture affects ALL connected users simultaneously
+// - Race conditions and culture conflicts will occur
+//
+// Migration Guide:
+// - Option 1: Use RequestLocalizationMiddleware for per-request culture
+// - Option 2: Use CascadingParameter with per-circuit culture state
+// - See: https://learn.microsoft.com/aspnet/core/blazor/globalization-localization
+//
+// Related files to update on migration:
+// - LanguageSelector.razor (remove manual culture setting)
+// - App.razor (may need CascadingValue for culture)
+// - Startup/Program.cs (add middleware)
 var cultureService = host.Services.GetRequiredService<CultureService>();
 var culture = await cultureService.InitializeCultureAsync();
 CultureInfo.DefaultThreadCurrentCulture = culture;