fix(playlists): surface bulk-delete button in header while multi-select is active (#290)

Selecting playlists via the header toggle only exposed the delete action
through the right-click context menu. Add a visible Delete-selected button
next to Cancel that kicks off the same handleDeleteSelected flow, with a
tooltip hint when some of the selected playlists aren't deletable (foreign
owner). Button is only rendered when selection mode is on and at least one
playlist is selected.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/locales/nl.ts

@@ -1266,6 +1266,7 @@ export const nlTranslation = {
     deleteSuccess: '{{count}} playlists verwijderd',
     deleteFailed: 'Fout bij verwijderen van {{name}}',
     deleteSelected: 'Geselecteerde verwijderen',
+    deleteSelectedPartial: 'Slechts {{n}} van de {{total}} geselecteerde playlists kunnen worden verwijderd (de andere zijn van iemand anders).',
     mergeSuccess: '{{count}} nummers samengevoegd in {{playlist}}',
     mergeNoNewSongs: 'Geen nieuwe nummers om toe te voegen',
     mergeError: 'Fout bij samenvoegen van playlists',