refactor(surface): drop the deprecated main-tab aliases and dead diagram surface

MainTab/activeMainTab/setActiveMainTab/setMainTabGuard were deprecated
mirrors of the surface names — every call site now uses
activeSurface/setActiveSurface/setSurfaceGuard directly and the aliases
are gone, including the persisted mirror field.

The 'diagram' surface had no way to open it (navigateToDiagram had no
callers except a .drawio attachment click that navigated to a surface
nothing rendered); the surface, DiagramView, and its store plumbing are
removed, and a .drawio attachment now opens in the file panel.

?tab= deep links map to the matching context-panel surface instead of
setting a main-area surface nothing renders, and a persisted non-chat
surface can no longer rehydrate into a blank main area.

packages/ui/src/components/session/project-context/useProjectTodoSend.ts

@@ -44,13 +44,13 @@ export const useProjectTodoSend = (options: {
   const sendMessage = useSessionUIStore((state) => state.sendMessage);
   const setCurrentSession = useSessionUIStore((state) => state.setCurrentSession);
   const setPendingInputText = useInputStore((state) => state.setPendingInputText);
-  const setActiveMainTab = useUIStore((state) => state.setActiveMainTab);
+  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const setSessionSwitcherOpen = useUIStore((state) => state.setSessionSwitcherOpen);
 
   const routeToChat = React.useCallback(() => {
-    setActiveMainTab('chat');
+    setActiveSurface('chat');
     setSessionSwitcherOpen(false);
-  }, [setActiveMainTab, setSessionSwitcherOpen]);
+  }, [setActiveSurface, setSessionSwitcherOpen]);
 
   const sendToCurrentSession = React.useCallback(
     (todoText: string) => {