Extend download information with source, timestamp, and user details

[Copilot]

This PR extends the dataset download information to provide better traceability and user understanding as requested in issue #2147.

Changes Made

1. Renamed manifest file for clarity

• Changed filename from manifest.json to general_metadata.json for better user comprehension
• Updated file generation and archive inclusion in ExportController.cs

2. Added download metadata fields

Extended the ApiDatasetModel with three new properties:

• DownloadSource: The base URL of the BEXIS2 instance (e.g., "https://www.bexis.uni-jena.de/")
• DownloadDateTime: Timestamp when the download was initiated
• DownloadedBy: Username of the authenticated user or "anonymous" for unauthenticated downloads

3. Implementation details

• Download source extracted from HttpContext.Current.Request.Url.GetLeftPart(UriPartial.Authority)
• Download timestamp uses DateTime.Now with consistent en-US culture formatting
• User information retrieved from HttpContext.Current.User.Identity.Name
• Comprehensive error handling with graceful fallbacks (e.g., "unknown" for URL, "anonymous" for user)

4. Example output

The enhanced general_metadata.json now includes:

{
  "Id": 123,
  "Version": 1,
  "Title": "Sample Dataset",
  "DownloadSource": "https://www.bexis.uni-jena.de/",
  "DownloadDateTime": "8/18/2025 9:08:50 PM", 
  "DownloadedBy": "john.doe",
  ...existing fields...
}

Backward Compatibility

All changes are purely additive - existing functionality remains unchanged and API consumers will continue to work without modification. The new fields provide additional valuable metadata without breaking existing workflows.

Files Modified

• Console/BExIS.Web.Shell/Areas/DIM/BExIS.Modules.Dim.UI/Models/API/ApiModels.cs
• Console/BExIS.Web.Shell/Areas/DIM/BExIS.Modules.Dim.UI/Helpers/ApiDatasetHelper.cs
• Console/BExIS.Web.Shell/Areas/DIM/BExIS.Modules.Dim.UI/Controllers/ExportController.cs

Fixes #2147.

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