Replaces the 86-line duplicated inline add-relationship form with
<AddRelationshipForm onSubmit={handleAddRelationship}>. The {#key node.id}
wrapper resets the form's open state when the selected tree node changes.
Year inputs now have <label> elements (WCAG 1.3.1) via the shared component.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>