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/pages/Playlists.tsx

@@ -734,6 +734,23 @@ export default function Playlists() {
               )}
             </>
           )}
+          {selectionMode && selectedIds.size > 0 && (() => {
+            const deletableCount = selectedPlaylists.filter(isPlaylistDeletable).length;
+            return (
+              <button
+                className="btn btn-danger"
+                onClick={handleDeleteSelected}
+                disabled={deletableCount === 0}
+                data-tooltip={deletableCount === selectedIds.size
+                  ? undefined
+                  : t('playlists.deleteSelectedPartial', { n: deletableCount, total: selectedIds.size })}
+                data-tooltip-pos="bottom"
+              >
+                <Trash2 size={15} />
+                {t('playlists.deleteSelected')}
+              </button>
+            );
+          })()}
           <button
             className={`btn btn-surface${selectionMode ? ' btn-sort-active' : ''}`}
             onClick={toggleSelectionMode}