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/App.tsx

@@ -626,7 +626,7 @@ function App({ apis }: AppProps) {
       const directory = typeof detail?.directory === 'string' && detail.directory.trim().length > 0
         ? detail.directory.trim()
         : null;
-      useUIStore.getState().setActiveMainTab('chat');
+      useUIStore.getState().setActiveSurface('chat');
       void useSessionUIStore.getState().setCurrentSession(sessionId, directory);
     };
 
@@ -675,7 +675,7 @@ function App({ apis }: AppProps) {
         ? detail.projectId.trim()
         : null;
       const hasProjectTarget = Boolean(directory || projectId);
-      useUIStore.getState().setActiveMainTab('chat');
+      useUIStore.getState().setActiveSurface('chat');
       useUIStore.getState().setSessionSwitcherOpen(false);
       useSessionUIStore.getState().openNewSessionDraft({
         target: hasProjectTarget ? 'project' : 'chat',