feat(web): add ACP chat panel with tabs (Terminal | Agent)

- web: add react-markdown + remark-gfm + react-syntax-highlighter
- web: new lib/api/acp.ts (useAcpStatus/History/Prompt/Cancel)
- web: new components/acp/ (AcpPanel + MessageBubble + MarkdownView +
  ThinkingIndicator). Markdown rendering with code highlighting via
  Prism oneDark. Auto-scroll to bottom when near it; error toasts
  fade after 5s.
- web: WorkspaceShell now renders a tabbed bottom panel (Terminal |
  Agent) when the bottom panel is visible. Tab state in
  workspace-ui-store.
- web: StatusBar shows agent status (ready / initializing / error)
  when the ACP session is initialized.
- web: pnpm-workspace.yaml: add packages ['.'] so pnpm install works
  in this workspace.

Conversation: 019f3562-9382-7e00-ad92-3162341e9274

web/package.json

@@ -29,8 +29,11 @@
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "react-hook-form": "^7.60.0",
+    "react-markdown": "^10.1.0",
     "react-resizable-panels": "^3.0.2",
     "react-router": "^7.7.0",
+    "react-syntax-highlighter": "^16.1.1",
+    "remark-gfm": "^4.0.1",
     "tailwind-merge": "^3.3.0",
     "tw-animate-css": "^1.3.4",
     "zod": "^4.0.0",
@@ -42,6 +45,7 @@
     "@types/node": "^24.0.10",
     "@types/react": "^19.1.8",
     "@types/react-dom": "^19.1.6",
+    "@types/react-syntax-highlighter": "^15.5.13",
     "@vitejs/plugin-react": "^4.6.2",
     "eslint": "^9.30.1",
     "eslint-plugin-react-hooks": "^5.2.0",