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/ScheduledTasksDialog.tsx

@@ -370,7 +370,7 @@ export function ScheduledTasksDialog() {
     setOpen(false);
     if (isMobile) {
       useFilesViewTabsStore.getState().setSelectedPath(selectedProject.path, task.loopFile, { allowOutsideRoot: true });
-      useUIStore.getState().setActiveMainTab('files');
+      useUIStore.getState().setActiveSurface('files');
       return;
     }
     useUIStore.getState().openContextFile(selectedProject.path, task.loopFile);
@@ -397,7 +397,7 @@ export function ScheduledTasksDialog() {
         // this surface (MainLayout closes surfaces on session selection).
         const project = projects.find((entry) => entry.id === selectedProjectID);
         useSessionUIStore.getState().setCurrentSession(sessionId, project?.path ?? null);
-        useUIStore.getState().setActiveMainTab('chat');
+        useUIStore.getState().setActiveSurface('chat');
       }
     } catch (error) {
       toast.error(error instanceof Error ? error.message : t('sessions.scheduledTasks.dialog.toast.runFailed'));