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/de.ts

@@ -1279,6 +1279,7 @@ export const deTranslation = {
     deleteSuccess: '{{count}} Playlists gelöscht',
     deleteFailed: 'Fehler beim Löschen von {{name}}',
     deleteSelected: 'Ausgewählte löschen',
+    deleteSelectedPartial: 'Nur {{n}} von {{total}} ausgewählten Playlists können gelöscht werden (die anderen gehören jemand anderem).',
     mergeSuccess: '{{count}} Songs in {{playlist}} zusammengeführt',
     mergeNoNewSongs: 'Keine neuen Songs zum Hinzufügen',
     mergeError: 'Fehler beim Zusammenführen von Playlists',