refactor(surface): remove the main-area surface concept entirely

activeSurface was permanently 'chat' after the legacy mobile layout
removal, so the whole concept is gone: the store field, surfaceGuard,
setActiveSurface/setSurfaceGuard, the per-runtime surface memory in
prepare/restoreForRuntimeSwitch, and WorkspaceSurface itself. All ~30
setActiveSurface('chat') call sites were no-ops and are deleted;
always-true 'is the chat active' checks in keyboard shortcuts, Header
and ChatContainer are unconditional now. FilesView's dirty-file guard
kept its file-switch and close protection but drops the surface-switch
branch nothing could trigger. TerminalView visibility comes only from
its callers. The router keeps parsing legacy ?tab= links (they open the
matching context-panel surface) via its own RouteTab type and no longer
serializes a tab or diff file into URLs — desktop URLs never carried
them anyway.

packages/ui/src/components/session/DirectoryExplorerDialog.tsx

@@ -148,7 +148,6 @@ export const DirectoryExplorerDialog: React.FC<DirectoryExplorerDialogProps> = (
   const homeDirectory = useDirectoryStore((s) => s.homeDirectory);
   const projects = useProjectsStore((s) => s.projects);
   const addProject = useProjectsStore((s) => s.addProject);
-  const setActiveSurface = useUIStore((s) => s.setActiveSurface);
   const setSessionSwitcherOpen = useUIStore((s) => s.setSessionSwitcherOpen);
   const openNewSessionDraft = useSessionUIStore((s) => s.openNewSessionDraft);
   const gitIdentityProfiles = useGitIdentitiesStore((s) => s.profiles);
@@ -411,11 +410,10 @@ export const DirectoryExplorerDialog: React.FC<DirectoryExplorerDialogProps> = (
   }, [onOpenChange]);
 
   const openProjectDraft = React.useCallback((projectId: string, projectPath: string) => {
-    setActiveSurface('chat');
     if (isMobile) setSessionSwitcherOpen(false);
     openNewSessionDraft({ selectedProjectId: projectId, directoryOverride: projectPath });
     handleClose();
-  }, [handleClose, isMobile, openNewSessionDraft, setActiveSurface, setSessionSwitcherOpen]);
+  }, [handleClose, isMobile, openNewSessionDraft, setSessionSwitcherOpen]);
 
   const handleQuickAdd = React.useCallback((event: React.MouseEvent, path: string) => {
     event.stopPropagation();