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/project-context/useProjectTodoSend.ts

@@ -44,13 +44,11 @@ export const useProjectTodoSend = (options: {
   const sendMessage = useSessionUIStore((state) => state.sendMessage);
   const setCurrentSession = useSessionUIStore((state) => state.setCurrentSession);
   const setPendingInputText = useInputStore((state) => state.setPendingInputText);
-  const setActiveSurface = useUIStore((state) => state.setActiveSurface);
   const setSessionSwitcherOpen = useUIStore((state) => state.setSessionSwitcherOpen);
 
   const routeToChat = React.useCallback(() => {
-    setActiveSurface('chat');
     setSessionSwitcherOpen(false);
-  }, [setActiveSurface, setSessionSwitcherOpen]);
+  }, [setSessionSwitcherOpen]);
 
   const sendToCurrentSession = React.useCallback(
     (todoText: string) => {