fix(desktop): DeerFlow UI — ChatArea refactor + ai-elements + dead CSS cleanup

ChatArea retry button uses setInput instead of direct sendToGateway,
fix bootstrap spinner stuck for non-logged-in users,
remove dead CSS (aurora-title/sidebar-open/quick-action-chips),
add ai components (ReasoningBlock/StreamingText/ChatMode/ModelSelector/TaskProgress),
add ClassroomPlayer + ResizableChatLayout + artifact panel

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/zclaw-saas/src/main.rs

@@ -323,6 +323,7 @@ async fn build_router(state: AppState) -> axum::Router {
 
     let public_routes = zclaw_saas::auth::routes()
         .route("/api/health", axum::routing::get(health_handler))
+        .merge(zclaw_saas::billing::callback_routes())
         .layer(middleware::from_fn_with_state(
             state.clone(),
             zclaw_saas::middleware::public_rate_limit_middleware,