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/hooks/useMenuActions.ts

@@ -102,7 +102,7 @@ export const useMenuActions = (
   const toggleHelpDialog = useUIStore((s) => s.toggleHelpDialog);
   const toggleSidebar = useUIStore((s) => s.toggleSidebar);
   const setSessionSwitcherOpen = useUIStore((s) => s.setSessionSwitcherOpen);
-  const setActiveMainTab = useUIStore((s) => s.setActiveMainTab);
+  const setActiveSurface = useUIStore((s) => s.setActiveSurface);
   const setSettingsDialogOpen = useUIStore((s) => s.setSettingsDialogOpen);
   const setAboutDialogOpen = useUIStore((s) => s.setAboutDialogOpen);
   const checkForUpdates = useUpdateStore((state) => state.checkForUpdates);
@@ -151,10 +151,10 @@ export const useMenuActions = (
     const nextSession = sessions[nextIndex];
     if (!nextSession) return;
 
-    setActiveMainTab('chat');
+    setActiveSurface('chat');
     setSessionSwitcherOpen(false);
     useSessionUIStore.getState().setCurrentSession(nextSession.id);
-  }, [setActiveMainTab, setSessionSwitcherOpen]);
+  }, [setActiveSurface, setSessionSwitcherOpen]);
 
   const navigateProject = React.useCallback((direction: -1 | 1) => {
     const { activeProjectId, projects, setActiveProject } = useProjectsStore.getState();
@@ -191,7 +191,7 @@ export const useMenuActions = (
           break;
 
         case 'new-session':
-          setActiveMainTab('chat');
+          setActiveSurface('chat');
           setSessionSwitcherOpen(false);
           {
             const sessionState = useSessionUIStore.getState();
@@ -203,7 +203,7 @@ export const useMenuActions = (
           break;
 
         case 'new-worktree-session':
-          setActiveMainTab('chat');
+          setActiveSurface('chat');
           setSessionSwitcherOpen(false);
           createWorktreeSession();
           break;
@@ -341,7 +341,7 @@ export const useMenuActions = (
       onToggleMemoryDebug,
       openNewSessionDraft,
       setAboutDialogOpen,
-      setActiveMainTab,
+      setActiveSurface,
       setSessionSwitcherOpen,
       setCommandPaletteOpen,
       setSettingsDialogOpen,