[Feature]: Attachments gallery  #76

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

Today the External Objects page lists the alt text of each attachment ("Picture missing. No alternative text available." for all 419 pictures of the Subset026 sample), without thumbnails.

Acceptance criteria

• Thumbnails with a lightbox and download; each picture links to the requirement that uses it.

Steps to Reproduce

Load requirements-and-objects.reqifz or ProR_Traceability-Template-v1.0.reqif (reqifviewer.Tests/TestData) and look at the pages mentioned above.

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 R-10 of the viewer improvement plan. Size: S. Depends on: [Feature]: Show pictures inline in rich text  #71.