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/lib/router/serializeRoute.ts

@@ -1,4 +1,3 @@
-import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { isEmbeddedSessionChat } from '@/components/layout/contextPanelEmbeddedChat';
 import { ROUTE_PARAMS } from './types';
 
@@ -7,17 +6,10 @@ import { ROUTE_PARAMS } from './types';
  */
 export interface AppRouteState {
   sessionId: string | null;
-  tab: WorkspaceSurface;
   isSettingsOpen: boolean;
   settingsPath: string;
-  diffFile: string | null;
 }
 
-/**
- * Default tab when none is specified.
- */
-const DEFAULT_TAB: WorkspaceSurface = 'chat';
-
 /**
  * Serialize application state to URL search parameters.
  * Only includes parameters that differ from defaults to keep URLs clean.
@@ -38,15 +30,6 @@ function serializeRoute(state: AppRouteState): URLSearchParams {
     return params;
   }
 
-  // Tab - only include if not the default
-  if (state.tab !== DEFAULT_TAB) {
-    params.set(ROUTE_PARAMS.TAB, state.tab);
-  }
-
-  // Diff file - only include when on diff tab
-  if (state.tab === 'diff' && state.diffFile && state.diffFile.trim().length > 0) {
-    params.set(ROUTE_PARAMS.FILE, state.diffFile);
-  }
 
   return params;
 }