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/views/ArchiveView.tsx

@@ -28,7 +28,7 @@ export function ArchiveView(): React.ReactNode {
   const { t } = useI18n();
   const open = useUIStore((state) => state.isArchivePageOpen);
   const setOpen = useUIStore((state) => state.setArchivePageOpen);
-  const setActiveMainTab = useUIStore((state) => state.setActiveMainTab);
+  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const setCurrentSession = useSessionUIStore((state) => state.setCurrentSession);
   const unarchiveSession = useSessionUIStore((state) => state.unarchiveSession);
   const homeDirectory = useDirectoryStore((state) => state.homeDirectory);
@@ -86,9 +86,9 @@ export function ArchiveView(): React.ReactNode {
   const openSession = React.useCallback((session: Session) => {
     const directory = normalizePath(resolveGlobalSessionDirectory(session));
     setCurrentSession(session.id, directory ?? undefined);
-    setActiveMainTab('chat');
+    setActiveSurface('chat');
     setOpen(false);
-  }, [setActiveMainTab, setCurrentSession, setOpen]);
+  }, [setActiveSurface, setCurrentSession, setOpen]);
 
   const restoreSession = React.useCallback((session: Session) => {
     void unarchiveSession(session.id).then((success) => {