Add username/password auth and login UI

Endpoints/EndpointHelpers.cs

@@ -7,20 +7,15 @@ namespace GameList.Endpoints;
 
 internal static class EndpointHelpers
 {
-    public static async Task<Player> GetOrCreatePlayer(HttpContext ctx, AppDbContext db)
+    public static async Task<Player?> GetAuthenticatedPlayer(HttpContext ctx, AppDbContext db)
     {
         if (!ctx.Items.TryGetValue(Infrastructure.PlayerIdentityExtensions.PlayerCookieName, out var value) || value is not Guid playerId)
         {
-            throw new InvalidOperationException("Player cookie missing.");
+            return null;
         }
 
         var existing = await db.Players.FindAsync(playerId);
-        if (existing != null) return existing;
-
-        var player = new Player { Id = playerId };
-        db.Players.Add(player);
-        await db.SaveChangesAsync();
-        return player;
+        return existing;
     }
 
     public static async Task<Phase> GetPhase(AppDbContext db)