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/layout/Header.tsx

@@ -434,7 +434,7 @@ export const Header: React.FC = () => {
   const openContextOverview = useUIStore((state) => state.openContextOverview);
   const openContextPlan = useUIStore((state) => state.openContextPlan);
   const closeContextPanel = useUIStore((state) => state.closeContextPanel);
-  const activeMainTab = useUIStore((state) => state.activeMainTab);
+  const activeSurface = useUIStore((state) => state.activeSurface);
   const shortcutOverrides = useUIStore((state) => state.shortcutOverrides);
 
   const getCurrentModel = useConfigStore((state) => state.getCurrentModel);
@@ -612,7 +612,7 @@ export const Header: React.FC = () => {
   }, [setWorkStatusOverlayOpen, setWorkStatusPanelEnabled, workStatusOverlayOpen, workStatusPanelEnabled, workStatusPanelFits]);
   const showDesktopHeaderContextUsage = !isVSCode
     && !workStatusPanelVisible
-    && activeMainTab === 'chat'
+    && activeSurface === 'chat'
     && !!stableDesktopContextUsage
     && stableDesktopContextUsage.totalTokens > 0;
   const desktopHeaderDisplayPercentage = stableDesktopContextUsage && stableDesktopContextUsage.contextLimit > 0
@@ -1153,8 +1153,8 @@ export const Header: React.FC = () => {
   // Reset plan tab availability when session changes
   React.useEffect(() => {
     if (!planModeEnabled) {
-      if (useUIStore.getState().activeMainTab === 'plan') {
-        useUIStore.getState().setActiveMainTab('chat');
+      if (useUIStore.getState().activeSurface === 'plan') {
+        useUIStore.getState().setActiveSurface('chat');
       }
       return;
     }
@@ -1167,8 +1167,8 @@ export const Header: React.FC = () => {
     }
 
     // If plan is not available but user is on plan tab, switch them back to chat
-    if (!planTabAvailable && useUIStore.getState().activeMainTab === 'plan') {
-      useUIStore.getState().setActiveMainTab('chat');
+    if (!planTabAvailable && useUIStore.getState().activeSurface === 'plan') {
+      useUIStore.getState().setActiveSurface('chat');
     }
   }, [
     planModeEnabled,
@@ -1759,7 +1759,7 @@ export const Header: React.FC = () => {
             className={cn(desktopHeaderIconButtonClass, 'mr-1')}
             Icon={'picture-in-picture-2'}
           />
-          {activeMainTab === 'chat' && !isVSCode ? (
+          {activeSurface === 'chat' && !isVSCode ? (
             <Tooltip>
               <TooltipTrigger asChild>
                 <button