[Feature]: New home page: drag-and-drop upload and sample files  #67

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 home page is a bare file picker with Load, Cancel and Clear buttons. A first-time visitor without a ReqIF file at hand cannot see what the viewer does.

Acceptance criteria

• Blueprint File Upload with drag-and-drop, progress while parsing, and clear errors (too large, unsupported, not valid ReqIF).
• "Try a sample" buttons for the bundled samples, so a first-time visitor sees the viewer working.
• One sentence on privacy: where the file is processed and that it is not stored.
• Optional: recent files (names only) in browser storage.

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-01 of the viewer improvement plan. Size: M.