v1.3.0 - Detect file-in-use and other access errors

Adds proactive detection of common file-access problems and translates them into user-friendly messages. When an Excel workbook is already open (Compare, Infer, or writing the diff output), the app now names the process holding the file (via Restart Manager) and asks the user to close it, instead of surfacing a raw sharing-violation error. Missing files, empty paths, missing output folders, permission-denied, and parse errors are also reported with clear, actionable messages. Requires .NET 10 Desktop Runtime (win-x64).