Load paintings instantly and allow overwriting saves in the paint HUD

Load now applies the selected slot immediately instead of going through a
confirmation dialog. Save keeps its OK button enabled even when the name
already exists (the warning text stays, purely informational) and writes
through with overwrite, so re-saving under the same name replaces the slot.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scripts/MeshPainter.Persistence.cs

@@ -71,14 +71,15 @@ public partial class MeshPainter
     }
 
     /// <summary>
-    /// Writes the current painted maps to a new named slot. Returns false without writing if the painter
-    /// is not ready or a slot with this name already exists (no silent overwrite).
+    /// Writes the current painted maps to a named slot. Returns false without writing if the painter is
+    /// not ready. An existing slot of the same name is overwritten only when <paramref name="overwrite"/>
+    /// is true; otherwise the call fails rather than clobbering it.
     /// </summary>
-    public bool SavePaintAs(string name)
+    public bool SavePaintAs(string name, bool overwrite = false)
     {
         if (!_setupValid || string.IsNullOrWhiteSpace(name)) return false;
         name = name.Trim();
-        if (PaintExists(name)) return false;
+        if (!overwrite && PaintExists(name)) return false;
 
         try
         {