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/sidebar/hooks/useProjectSessionSelection.ts

@@ -2,7 +2,7 @@ import React from 'react';
 import type { Session } from '@opencode-ai/sdk/v2';
 import type { SessionGroup, SessionNode } from '../types';
 import { normalizePath } from '../utils';
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { useUIStore } from '@/stores/useUIStore';
 import { useSessionUIStore } from '@/sync/session-ui-store';
 
@@ -22,7 +22,7 @@ type Args = {
   newSessionDraftOpen: boolean;
   mobileVariant: boolean;
   openNewSessionDraft: (options?: { selectedProjectId?: string | null; directoryOverride?: string | null }) => void;
-  setActiveMainTab: (tab: MainTab) => void;
+  setActiveSurface: (tab: WorkspaceSurface) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
 };
 
@@ -101,7 +101,7 @@ export const useProjectSessionSelection = (args: Args): void => {
     newSessionDraftOpen,
     mobileVariant,
     openNewSessionDraft,
-    setActiveMainTab,
+    setActiveSurface,
     setSessionSwitcherOpen,
   } = args;
 
@@ -205,7 +205,7 @@ export const useProjectSessionSelection = (args: Args): void => {
     previousActiveProjectRef.current = activeProjectId;
 
     if (selection.kind === 'open-draft') {
-      setActiveMainTab('chat');
+      setActiveSurface('chat');
       if (mobileVariant) {
         setSessionSwitcherOpen(false);
       }
@@ -232,7 +232,7 @@ export const useProjectSessionSelection = (args: Args): void => {
     openNewSessionDraft,
     projectSections,
     projectSessionMeta,
-    setActiveMainTab,
+    setActiveSurface,
     setSessionSwitcherOpen,
     setActiveSessionByProject,
   ]);

packages/ui/src/components/session/sidebar/hooks/useSessionActions.ts

@@ -3,7 +3,7 @@ import type { Session } from '@opencode-ai/sdk/v2';
 import { toast } from '@/components/ui';
 import { copyTextToClipboard } from '@/lib/clipboard';
 import { useI18n } from '@/lib/i18n';
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { useUIStore } from '@/stores/useUIStore';
 import { streamPerfMark } from '@/stores/utils/streamDebug';
 import { useSessionUIStore } from '@/sync/session-ui-store';
@@ -30,7 +30,7 @@ type Args = {
   sessionSearchQuery: string;
   setSessionSearchQuery: (value: string) => void;
   setIsSessionSearchOpen: (open: boolean) => void;
-  setActiveMainTab: (tab: MainTab) => void;
+  setActiveSurface: (tab: WorkspaceSurface) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
   setCurrentSession: (sessionId: string | null, directoryHint?: string | null) => void;
   updateSessionTitle: (id: string, title: string) => Promise<void>;
@@ -79,7 +79,7 @@ export const useSessionActions = (args: Args) => {
       };
 
       if (args.mobileVariant) {
-        args.setActiveMainTab('chat');
+        args.setActiveSurface('chat');
         args.setSessionSwitcherOpen(false);
       }