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/sidebar/hooks/useProjectSessionSelection.ts

@@ -2,7 +2,6 @@ import React from 'react';
 import type { Session } from '@opencode-ai/sdk/v2';
 import type { SessionGroup, SessionNode } from '../types';
 import { normalizePath } from '../utils';
-import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { useUIStore } from '@/stores/useUIStore';
 import { useSessionUIStore } from '@/sync/session-ui-store';
 
@@ -22,7 +21,6 @@ type Args = {
   newSessionDraftOpen: boolean;
   mobileVariant: boolean;
   openNewSessionDraft: (options?: { selectedProjectId?: string | null; directoryOverride?: string | null }) => void;
-  setActiveSurface: (tab: WorkspaceSurface) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
 };
 
@@ -101,7 +99,6 @@ export const useProjectSessionSelection = (args: Args): void => {
     newSessionDraftOpen,
     mobileVariant,
     openNewSessionDraft,
-    setActiveSurface,
     setSessionSwitcherOpen,
   } = args;
 
@@ -205,7 +202,6 @@ export const useProjectSessionSelection = (args: Args): void => {
     previousActiveProjectRef.current = activeProjectId;
 
     if (selection.kind === 'open-draft') {
-      setActiveSurface('chat');
       if (mobileVariant) {
         setSessionSwitcherOpen(false);
       }
@@ -232,7 +228,6 @@ export const useProjectSessionSelection = (args: Args): void => {
     openNewSessionDraft,
     projectSections,
     projectSessionMeta,
-    setActiveSurface,
     setSessionSwitcherOpen,
     setActiveSessionByProject,
   ]);

packages/ui/src/components/session/sidebar/hooks/useSessionActions.ts

@@ -3,7 +3,6 @@ import type { Session } from '@opencode-ai/sdk/v2';
 import { toast } from '@/components/ui';
 import { copyTextToClipboard } from '@/lib/clipboard';
 import { useI18n } from '@/lib/i18n';
-import type { WorkspaceSurface } from '@/stores/useUIStore';
 import { useUIStore } from '@/stores/useUIStore';
 import { streamPerfMark } from '@/stores/utils/streamDebug';
 import { useSessionUIStore } from '@/sync/session-ui-store';
@@ -30,7 +29,6 @@ type Args = {
   sessionSearchQuery: string;
   setSessionSearchQuery: (value: string) => void;
   setIsSessionSearchOpen: (open: boolean) => void;
-  setActiveSurface: (tab: WorkspaceSurface) => void;
   setSessionSwitcherOpen: (open: boolean) => void;
   setCurrentSession: (sessionId: string | null, directoryHint?: string | null) => void;
   updateSessionTitle: (id: string, title: string) => Promise<void>;
@@ -79,7 +77,6 @@ export const useSessionActions = (args: Args) => {
       };
 
       if (args.mobileVariant) {
-        args.setActiveSurface('chat');
         args.setSessionSwitcherOpen(false);
       }