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/hooks/useMenuActions.ts

@@ -102,7 +102,6 @@ export const useMenuActions = (
   const toggleHelpDialog = useUIStore((s) => s.toggleHelpDialog);
   const toggleSidebar = useUIStore((s) => s.toggleSidebar);
   const setSessionSwitcherOpen = useUIStore((s) => s.setSessionSwitcherOpen);
-  const setActiveSurface = useUIStore((s) => s.setActiveSurface);
   const setSettingsDialogOpen = useUIStore((s) => s.setSettingsDialogOpen);
   const setAboutDialogOpen = useUIStore((s) => s.setAboutDialogOpen);
   const checkForUpdates = useUpdateStore((state) => state.checkForUpdates);
@@ -151,10 +150,9 @@ export const useMenuActions = (
     const nextSession = sessions[nextIndex];
     if (!nextSession) return;
 
-    setActiveSurface('chat');
     setSessionSwitcherOpen(false);
     useSessionUIStore.getState().setCurrentSession(nextSession.id);
-  }, [setActiveSurface, setSessionSwitcherOpen]);
+  }, [setSessionSwitcherOpen]);
 
   const navigateProject = React.useCallback((direction: -1 | 1) => {
     const { activeProjectId, projects, setActiveProject } = useProjectsStore.getState();
@@ -191,8 +189,7 @@ export const useMenuActions = (
           break;
 
         case 'new-session':
-          setActiveSurface('chat');
-          setSessionSwitcherOpen(false);
+                setSessionSwitcherOpen(false);
           {
             const sessionState = useSessionUIStore.getState();
             const directory = useDirectoryStore.getState().currentDirectory;
@@ -203,8 +200,7 @@ export const useMenuActions = (
           break;
 
         case 'new-worktree-session':
-          setActiveSurface('chat');
-          setSessionSwitcherOpen(false);
+                setSessionSwitcherOpen(false);
           createWorktreeSession();
           break;
 
@@ -341,7 +337,6 @@ export const useMenuActions = (
       onToggleMemoryDebug,
       openNewSessionDraft,
       setAboutDialogOpen,
-      setActiveSurface,
       setSessionSwitcherOpen,
       setCommandPaletteOpen,
       setSettingsDialogOpen,