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/SessionSwitcherDropdown.tsx

@@ -71,14 +71,14 @@ type SwitcherContentProps = {
 function SwitcherContent({ onSelect, variant, scopeProjectId }: SwitcherContentProps): React.ReactElement {
   const items = useSwitcherItems(true, { scopeProjectId });
   const openNewSessionDraft = useSessionUIStore((state) => state.openNewSessionDraft);
-  const setActiveMainTab = useUIStore((state) => state.setActiveMainTab);
+  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const { t } = useI18n();
 
   const handleNewSession = React.useCallback(() => {
-    setActiveMainTab('chat');
+    setActiveSurface('chat');
     onSelect();
     openNewSessionDraft();
-  }, [onSelect, openNewSessionDraft, setActiveMainTab]);
+  }, [onSelect, openNewSessionDraft, setActiveSurface]);
 
   const [expandedParents, setExpandedParents] = React.useState<Set<string>>(new Set());
   const toggleParent = React.useCallback((sessionId: string) => {