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/PlanView.tsx

@@ -168,7 +168,6 @@ export const PlanView: React.FC<PlanViewProps> = ({ targetPath = null, projectPl
   const activeProjectId = useProjectsStore((state) => state.activeProjectId);
   const gitDirectories = useGitStore((state) => state.directories);
   const effectiveDirectory = useEffectiveDirectory() ?? '';
-  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const setSessionSwitcherOpen = useUIStore((state) => state.setSessionSwitcherOpen);
   const runtimeApis = useRuntimeAPIs();
   const { isMobile } = useDeviceInfo();
@@ -579,10 +578,9 @@ export const PlanView: React.FC<PlanViewProps> = ({ targetPath = null, projectPl
   }, []);
 
   const routeToChat = React.useCallback(() => {
-    setActiveSurface('chat');
     setSessionSwitcherOpen(false);
     onNavigatedToChat?.();
-  }, [onNavigatedToChat, setActiveSurface, setSessionSwitcherOpen]);
+  }, [onNavigatedToChat, setSessionSwitcherOpen]);
 
   const handleConfirmPlanSend = React.useCallback(
     async (execution: TodoSendExecution) => {