feat(web): workspace components — ComparePane, Memory page, McpDialog, error boundaries, message-parts

- Add ComparePane.tsx: side-by-side AI response comparison
- Add Memory.tsx: memory management page with CRUD UI
- Add McpPermissionDialog.tsx: MCP tool permission approval dialog
- Add McpResponseDisplay.tsx: MCP response visualization
- Add MessageBoundary.tsx + MessageListErrorBoundary.tsx: error resilience
- Add EmptyState.tsx: contextual empty state component
- Add KeyboardShortcutsDialog.tsx: keyboard shortcut reference
- Add message-parts/: ActionRow, CompactCard, MistakeRecoverySentinel, ReasoningBlock, SendToTerminalMenu, StatsLine, SummaryCard
- Add useDraftPersistence.ts: draft message persistence hook
- Add useTerminals.ts: terminal session management hook
- Add keyboard-shortcuts.ts + tool-utils.ts: shared utilities
- Extend components: ChatInput, MessageBubble, MessageList, Workspace, panes
- Extend hooks: useTerminalSocket, useSessionStream test suite
- Update pages: Home, Project — workspace layout and session flow

apps/web/src/components/NewPaneMenu.tsx

@@ -1,5 +1,5 @@
 import { useState } from 'react';
-import { Code, History, MessageSquare, Plus, Terminal, Workflow } from 'lucide-react';
+import { Code, History, MessageSquare, Plus, Swords, Terminal, Workflow } from 'lucide-react';
 import { api } from '@/api/client';
 import type { FlowRunRow } from '@/api/types';
 import { sessionEvents } from '@/hooks/sessionEvents';
@@ -90,6 +90,19 @@ export function NewPaneMenu({ onAddPane, disabled, projectId }: Props) {
             <Workflow size={14} /> New Orchestrator
           </DropdownMenuItem>
         )}
+        {projectId && (
+          <DropdownMenuItem
+            onSelect={() =>
+              sessionEvents.emit({
+                type: 'open_arena_launcher',
+                project_id: projectId,
+                placement: 'new',
+              })
+            }
+          >
+            <Swords size={14} /> New Arena
+          </DropdownMenuItem>
+        )}
 
         {projectId && (
           <>