Multi-link selection mode for 3+ selected sites

[wilhel1812]

Summary\nWhen users select more than two sites, support an explicit multi-link selection mode instead of falling back to a single pair model.\n\n## Motivation\nIssue #104 introduces multi-site selection behavior and mode defaults. Multi-link path/profile behavior for 3+ selected sites should be handled in a dedicated pass to reduce regression risk.\n\n## Scope\n- Define selection semantics for multiple active site pairs\n- Define profile behavior for multi-link context\n- Define map/link highlighting and inspector affordances for multi-link workflows\n- Keep saved links visible/clickable and integrate with multi-link selection UX\n\n## Notes\n- Keep this issue in discussion until scope and UX are finalized\n- Related: #104

[wilhel1812]

Created to track deferred scope from #104. No implementation started in this pass beyond creating this issue and linking scope.