feat: Improve document management with Save/Save As and keyboard shortcuts

- Track file path after save/load so Save doesn't prompt again
- Add Save As (Ctrl+Shift+S) to always prompt for location
- Update window title to show current filename
- Generate incremental default filenames (CutList_1.json, etc.)
- Add keyboard shortcuts: Ctrl+S (Save), Ctrl+O (Open), Ctrl+N (New)
- Enter key in items grid moves to Length column on next row

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

CutList/Services/DocumentService.cs

@@ -41,6 +41,7 @@ namespace CutList.Services
             {
                 var json = File.ReadAllText(filePath);
                 var document = JsonConvert.DeserializeObject<Document>(json);
+                document.LastFilePath = filePath;
                 return Result<Document>.Success(document);
             }
             catch (Exception ex)