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/chat/ChatContainer.tsx

@@ -1006,8 +1006,8 @@ export const ChatContainer: React.FC<ChatContainerProps> = ({
                 return;
             }
 
-            const { activeMainTab } = useUIStore.getState();
-            if (activeMainTab !== 'chat' || hasBlockingChatOverlay()) {
+            const { activeSurface } = useUIStore.getState();
+            if (activeSurface !== 'chat' || hasBlockingChatOverlay()) {
                 return;
             }