refactor(examples): use slot render helpers

Use the slhx RenderSlotExt path for board updates so examples no longer manually compose SafeHtml before slot replacement.

req: dx/006

req: component/003

examples/kanban/src/main.rs

@@ -198,7 +198,7 @@ fn registry(state: Arc<AppState>) -> HandlerRegistry {
 
 fn board_effects(board: &BoardState, notice: &'static str) -> impl IntoEffect {
     (
-        slots::board.html(ui::render(&board_view(board))),
+        slots::board.render(&board_view(board)),
         slots::notice.text(notice),
         forms::create_card.clear("title"),
     )