[Feature]: Review changes and export a new version  #84

What type of issue is this?

• Bug report
• Feature request

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the reqifviewer
• I have searched open and closed issues to ensure it has not already been reported

Description

Acceptance criteria

• Before downloading, a summary of the changes (reusing the comparison of As a developer I'd like to interactively explore the requirement ID(s) and their parent/child dependencies which have been changed between two reqif file versions (aka "baseline diffing"). #41).
• Export as .reqif or .reqifz: identifiers stay stable, LastChange is updated only on changed elements, the header gets a new CreationTime and ReqIFToolId ("ReqIF Viewer ") while SourceToolId and the original namespaces are kept.
• The file is validated against the schema before download; if it fails, the problems are shown instead.

Steps to Reproduce

NA

System Configuration

• reqifviewer version: 6.1.0 (development, with [Refactor] Blazor Blueprint app shell and Blazor Web App hosting #56)
• Environment (Operating system, version and so on): Windows 11, Chrome
• .NET Framework version: .NET 10
• Additional information: Plan item E-06 of the viewer improvement plan. Size: M. Depends on: [Feature]: Write attachments (external objects) into a .reqifz and keep the original entry names  reqifsharp#84, [Feature]: Edit session with change tracking and undo  #81, As a developer I'd like to interactively explore the requirement ID(s) and their parent/child dependencies which have been changed between two reqif file versions (aka "baseline diffing"). #41.