feat(workspace): deleted folders go to the trash as a whole, after a confirmation

A deleted personal folder is kept in trashed_folders with its subfolders;
its notes are trashed and tied to it. The trash lists the folder as one
entry with its note count; restoring brings back the folder tree (empty
subfolders too) and all notes, deleting for good, emptying the trash and
the notes trash delay remove it with its notes. Folders in shared folders
keep the old way.

Deleting a folder now asks first, in a small popover at the click
(ConfirmPopover, documented in ui-patterns).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/storage/retention.go

@@ -207,6 +207,13 @@ func (s *Store) PurgeExpired(now time.Time, allowNever bool) (PurgeResult, error
 		}
 		res.Notes = append(res.Notes, id)
 	}
+	// Folders in the notes trash: same delay; their notes went above (they were trashed at the same time)
+	if _, err := s.db.Exec(`DELETE FROM trashed_folders WHERE id IN (
+		SELECT tf.id FROM trashed_folders tf JOIN users u ON u.id = tf.user_id
+		WHERE `+daysSQL("u.note_trash_days")+` > 0 AND `+cutoffSQL("tf.trashed_at", daysSQL("u.note_trash_days"))+`)`,
+		neverAs(allowNever, d.NoteTrash), since, since, sqlNow(now), neverAs(allowNever, d.NoteTrash)); err != nil {
+		return res, err
+	}
 
 	// Ticked-off list items: the list owner's delay, from ticking them off
 	rows, err = s.db.Query(`SELECT li.id, li.list_id FROM list_items li JOIN lists l ON l.id = li.list_id JOIN users u ON u.id = l.user_id