fix: add missing System.Numerics import for Vector2 in ComposeBlueprintContainer

The upstream merge introduced new Vector2 usages (lines 206, 211) for
new combo button icons without the corresponding System.Numerics import,
which was required after our osuTK → System.Numerics.Vector2 migration.

Agent-Logs-Url: https://github.com/winnerspiros/osu/sessions/c574fc49-f3fa-4386-9706-f05472318292

Co-authored-by: winnerspiros <1675249+winnerspiros@users.noreply.github.com>

Author: Copilot

osu.Game/Screens/Edit/Compose/Components/ComposeBlueprintContainer.cs

@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Numerics;
 using Humanizer;
 using JetBrains.Annotations;
 using osu.Framework.Allocation;