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;
             }
 

packages/ui/src/components/chat/FileAttachment.tsx

@@ -2,6 +2,7 @@ import React, { useRef, memo } from 'react';
 import { useInputStore } from '@/sync/input-store';
 import type { AttachedFile } from '@/sync/session-ui-store';
 import { useUIStore } from '@/stores/useUIStore';
+import { useDirectoryStore } from '@/stores/useDirectoryStore';
 import { toast } from '@/components/ui';
 import { cn } from '@/lib/utils';
 import { openExternalUrl } from '@/lib/url';
@@ -833,7 +834,10 @@ export const MessageFilesDisplay = memo(({ files, onShowPopup, compact = false }
                 <button
                   type="button"
                   onClick={() => {
-                    useUIStore.getState().navigateToDiagram(filePath);
+                    const directory = useDirectoryStore.getState().currentDirectory;
+                    if (directory) {
+                      useUIStore.getState().openContextFile(directory, filePath);
+                    }
                   }}
                   className={cn(
                     "flex items-center gap-2 p-2 rounded-lg border border-border/40 bg-muted/10 hover:bg-muted/20 transition-colors text-left cursor-pointer",