[Feature]: Traceability: relation graph and a better relation matrix  #74

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

• From a requirement, a graph of its traces one or two levels deep, coloured by relation type.
• Relation matrix: choose source and target specification and relation types, readable labels, and cells that open the relation.
• "Coverage" figures: requirements without incoming or outgoing traces (feeds [Feature]: Document overview instead of Statistics  #75).

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 R-08 of the viewer improvement plan. Size: L. Depends on: [Feature]: Readable labels for requirements: detect the id and text attributes  #68.