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/apps/runtimeEndpointReset.ts

@@ -6,7 +6,6 @@ import { useProjectsStore } from '@/stores/useProjectsStore';
 import { useProjectContextStore } from '@/stores/useProjectContextStore';
 import { useGlobalSessionsStore } from '@/stores/useGlobalSessionsStore';
 import { useAutoReviewStore } from '@/stores/useAutoReviewStore';
-import { useUIStore } from '@/stores/useUIStore';
 import { usePermissionStore } from '@/stores/permissionStore';
 import { useFileSearchStore } from '@/stores/useFileSearchStore';
 import { useGitStore } from '@/stores/useGitStore';
@@ -37,7 +36,6 @@ export const reconnectAppForTransportSwitch = (): void => {
 
 export const resetAppForRuntimeEndpointChange = (detail: RuntimeEndpointChangedDetail): void => {
   useSessionUIStore.getState().prepareForRuntimeSwitch(detail.previousRuntimeKey);
-  useUIStore.getState().prepareForRuntimeSwitch(detail.previousRuntimeKey);
   if (detail.previousRuntimeKey) {
     useAutoReviewStore.getState().stopRunningRunsForRuntime(detail.previousRuntimeKey);
   }
@@ -71,7 +69,6 @@ export const resetAppForRuntimeEndpointChange = (detail: RuntimeEndpointChangedD
   useSessionFoldersStore.getState().resetForRuntimeSwitch(detail.runtimeKey);
   useFilesViewTabsStore.getState().resetForRuntimeSwitch(detail.runtimeKey);
   useSessionUIStore.getState().restoreForRuntimeSwitch(detail.runtimeKey);
-  useUIStore.getState().restoreForRuntimeSwitch(detail.runtimeKey);
   resetStreamingState();
   queueMicrotask(() => void syncDesktopSettings());
 };