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/SessionGroupSection.tsx

@@ -14,7 +14,7 @@ import { Button } from '@/components/ui/button';
 import { Icon } from "@/components/icon/Icon";
 import { cn } from '@/lib/utils';
 import { sessionEvents } from '@/lib/sessionEvents';
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { SessionFolderItem } from '../SessionFolderItem';
 import type { SortableDragHandleProps } from './sortableItems';
 import { DroppableFolderWrapper, SessionFolderDndScope } from './sessionFolderDnd';
@@ -84,7 +84,7 @@ type Props = {
   alwaysShowActions: boolean;
   activeProjectId: string | null;
   setActiveProjectIdOnly: (id: string) => void;
-  setActiveMainTab: (tab: MainTab) => void;
+  setActiveSurface: (tab: WorkspaceSurface) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
   openNewSessionDraft: (options?: { selectedProjectId?: string | null; directoryOverride?: string | null; targetFolderId?: string; target?: 'chat' | 'project' }) => void;
   addSessionToFolder: (scopeKey: string, folderId: string, sessionId: string) => void;
@@ -265,7 +265,7 @@ const areGroupPropsEqual = (prev: Props, next: Props): boolean => {
     && prev.alwaysShowActions === next.alwaysShowActions
     && prev.activeProjectId === next.activeProjectId
     && prev.setActiveProjectIdOnly === next.setActiveProjectIdOnly
-    && prev.setActiveMainTab === next.setActiveMainTab
+    && prev.setActiveSurface === next.setActiveSurface
     && prev.setSessionSwitcherOpen === next.setSessionSwitcherOpen
     && prev.openNewSessionDraft === next.openNewSessionDraft
     && prev.addSessionToFolder === next.addSessionToFolder
@@ -307,7 +307,7 @@ function SessionGroupSectionBase(props: Props): React.ReactNode {
     alwaysShowActions,
     activeProjectId,
     setActiveProjectIdOnly,
-    setActiveMainTab,
+    setActiveSurface,
     setSessionSwitcherOpen,
     openNewSessionDraft,
     addSessionToFolder,
@@ -880,7 +880,7 @@ function SessionGroupSectionBase(props: Props): React.ReactNode {
             depth={0}
             onNewSession={() => {
               if (projectId && projectId !== activeProjectId) setActiveProjectIdOnly(projectId);
-              setActiveMainTab('chat');
+              setActiveSurface('chat');
               if (mobileVariant) setSessionSwitcherOpen(false);
                openNewSessionDraft({
                  selectedProjectId: projectId,
@@ -1248,7 +1248,7 @@ function SessionGroupSectionBase(props: Props): React.ReactNode {
                   onClick={(event) => {
                     event.stopPropagation();
                     if (projectId && projectId !== activeProjectId) setActiveProjectIdOnly(projectId);
-                    setActiveMainTab('chat');
+                    setActiveSurface('chat');
                     if (mobileVariant) setSessionSwitcherOpen(false);
                     openNewSessionDraft({ selectedProjectId: projectId, directoryOverride: group.directory });
                   }}

packages/ui/src/components/session/sidebar/SidebarProjectsList.tsx

@@ -16,7 +16,7 @@ import type { SortableDragHandleProps } from './sortableItems';
 import { ProjectHeaderIdentity, SortableGroupItem, SortableProjectItem } from './sortableItems';
 import { formatProjectLabel } from './utils';
 import { useI18n } from '@/lib/i18n';
-import type { MainTab } from '@/stores/useUIStore';
+import type { WorkspaceSurface } from '@/stores/useUIStore';
 import type { ProjectSortOrder } from '@/stores/useSessionDisplayStore';
 import { streamPerfCount } from '@/stores/utils/streamDebug';
 import { Icon } from '@/components/icon/Icon';
@@ -91,7 +91,7 @@ type Props = {
   alwaysShowActions: boolean;
   toggleProject: (id: string) => void;
   setActiveProjectIdOnly: (id: string) => void;
-  setActiveMainTab: (tab: MainTab) => void;
+  setActiveSurface: (tab: WorkspaceSurface) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
   openNewSessionDraft: (options?: { selectedProjectId?: string | null; directoryOverride?: string | null }) => void;
   openNewWorktreeDialog: () => void;
@@ -362,7 +362,7 @@ function SidebarProjectsListComponent(props: Props): React.ReactNode {
                   }}
                   onNewSession={() => {
                     if (projectKey !== props.activeProjectId) props.setActiveProjectIdOnly(projectKey);
-                    props.setActiveMainTab('chat');
+                    props.setActiveSurface('chat');
                     if (props.mobileVariant) props.setSessionSwitcherOpen(false);
                     props.openNewSessionDraft({
                       selectedProjectId: projectKey,
@@ -371,7 +371,7 @@ function SidebarProjectsListComponent(props: Props): React.ReactNode {
                   }}
                   onNewWorktreeSession={() => {
                     if (projectKey !== props.activeProjectId) props.setActiveProjectIdOnly(projectKey);
-                    props.setActiveMainTab('chat');
+                    props.setActiveSurface('chat');
                     props.openNewWorktreeDialog();
                   }}
                   onManageWorktrees={() => props.openWorktreesPage(projectKey)}

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);
       }