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/ScheduledTasksDialog.tsx

@@ -368,13 +368,9 @@ export function ScheduledTasksDialog() {
       return;
     }
     setOpen(false);
-    if (isMobile) {
-      useFilesViewTabsStore.getState().setSelectedPath(selectedProject.path, task.loopFile, { allowOutsideRoot: true });
-      useUIStore.getState().setActiveSurface('files');
-      return;
-    }
+    useFilesViewTabsStore.getState().setSelectedPath(selectedProject.path, task.loopFile, { allowOutsideRoot: true });
     useUIStore.getState().openContextFile(selectedProject.path, task.loopFile);
-  }, [isMobile, selectedProject?.path, setOpen]);
+  }, [selectedProject?.path, setOpen]);
 
   const handleRunNow = React.useCallback(async (task: ScheduledTask) => {
     if (!selectedProjectID) {
@@ -397,8 +393,7 @@ export function ScheduledTasksDialog() {
         // this surface (MainLayout closes surfaces on session selection).
         const project = projects.find((entry) => entry.id === selectedProjectID);
         useSessionUIStore.getState().setCurrentSession(sessionId, project?.path ?? null);
-        useUIStore.getState().setActiveSurface('chat');
-      }
+        }
     } catch (error) {
       toast.error(error instanceof Error ? error.message : t('sessions.scheduledTasks.dialog.toast.runFailed'));
     } finally {