chore(PepLib.Core): remove unused using directives and clean up formatting

Remove unnecessary System, System.Collections.Generic, System.IO, and
System.Linq using directives that were flagged by IDE analyzers. Also
includes minor whitespace and code style normalization.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

PepLib.Core/Data/PlateHeader.cs

@@ -1,10 +1,9 @@
 namespace PepLib.Data
 {
     using System;
-    using System.Collections.Generic;
     using System.ComponentModel.DataAnnotations;
     using System.ComponentModel.DataAnnotations.Schema;
-    
+
 
     [Table("PlateHeader")]
     public partial class PlateHeader