Add phase-aware frontend shell, polling client, and global exception handler

Program.cs

@@ -1,5 +1,6 @@
 using GameList.Data;
 using GameList.Domain;
+using Microsoft.AspNetCore.Diagnostics;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.Data.Sqlite;
@@ -46,6 +47,23 @@ builder.Services.ConfigureHttpJsonOptions(options =>
 
 var app = builder.Build();
 
+app.UseExceptionHandler(handler =>
+{
+    handler.Run(async context =>
+    {
+        var feature = context.Features.Get<IExceptionHandlerFeature>();
+        var logger = context.RequestServices.GetRequiredService<ILoggerFactory>().CreateLogger("GlobalException");
+        if (feature?.Error != null)
+        {
+            logger.LogError(feature.Error, "Unhandled exception");
+        }
+
+        context.Response.StatusCode = StatusCodes.Status500InternalServerError;
+        context.Response.ContentType = "application/json";
+        await context.Response.WriteAsJsonAsync(new { error = "Unexpected server error" });
+    });
+});
+
 // Ensure database and migrations are applied on startup
 using (var scope = app.Services.CreateScope())
 {