Add] selected element summary card in Model Editor right panel; fixes #768

[samatstariongroup]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the COMET-WEB code style guidelines
• I have provided test coverage for my change (where applicable)

Description

• COMETwebapp/Components/ModelEditor/DetailsPanelEditor.razor: prepended a summary card showing Name, ShortName, Owner pill, category pills (when present), and Definition text (when present), gated by the existing SelectedSystemNode != null check; added @using COMET.Web.Common.Extensions for AsCommaSeparated(). The existing parameter CardView renders below it unchanged.
• COMETwebapp.Tests/Components/ModelEditor/DetailsPanelEditorTestFixture.cs (new) — 5 tests: full-card render, null selection, no-categories edge case, no-definition edge case, ElementUsage inheriting categories from its ElementDefinition via GetAllCategories().