[Add] delete ElementUsage and ElementDefinition in Model Editor; fixes #771 and #772

[samatstariongroup]

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Description

#771 — Web-app-friendly delete for ElementDefinition / ElementUsage

• Danger-styled Delete button in the details-panel action header (visible only with selection; disabled with tooltip when the selected element is the iteration's
  TopElement).
• Reuses ConfirmCancelPopup with a uniform message — no reference-count warning (server cascades cleanup of referencing usages).
• ModelEditorViewModel exposes SelectedElement, IsSelectedElementTopElement, IsOnDeletionMode, DeleteElementPopupViewModel, OpenDeleteElementPopup,
  DeleteSelectedElementAsync. Delete clones container + thing, calls SessionService.DeleteThingsWithNotification, clears selection on success, and finally-closes the popup
  so a thrown exception can no longer leave the dialog open. ILogger injected for error logging.