Add agent workflow engine foundation and theme-style page switcher.

Introduce WorkflowEngine with state machine, tool registry, and builtin chat template; migrate stream chat to engine callbacks. Move page mode switching to TopBar actions cluster as a ThemeToggle-style dropdown (聊天/工作室/爽文/房间).

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/index.css

@@ -27,6 +27,10 @@
   margin-top: 0; /* Ensure panels start from top */
 }
 
+.main-container.placeholder-container {
+  flex-direction: column;
+}
+
 /* Smooth transitions for theme changes - only apply to specific properties */
 .app {
   transition: background-color var(--transition-normal),