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/views/ArchiveView.tsx

@@ -28,7 +28,6 @@ export function ArchiveView(): React.ReactNode {
   const { t } = useI18n();
   const open = useUIStore((state) => state.isArchivePageOpen);
   const setOpen = useUIStore((state) => state.setArchivePageOpen);
-  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const setCurrentSession = useSessionUIStore((state) => state.setCurrentSession);
   const unarchiveSession = useSessionUIStore((state) => state.unarchiveSession);
   const homeDirectory = useDirectoryStore((state) => state.homeDirectory);
@@ -86,9 +85,8 @@ export function ArchiveView(): React.ReactNode {
   const openSession = React.useCallback((session: Session) => {
     const directory = normalizePath(resolveGlobalSessionDirectory(session));
     setCurrentSession(session.id, directory ?? undefined);
-    setActiveSurface('chat');
     setOpen(false);
-  }, [setActiveSurface, setCurrentSession, setOpen]);
+  }, [setCurrentSession, setOpen]);
 
   const restoreSession = React.useCallback((session: Session) => {
     void unarchiveSession(session.id).then((success) => {