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/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)}