feat: Phase 0 — monorepo 骨架 + 全表迁移 + FastAPI/Next 骨架 + CI

- uv(workspace) + pnpm monorepo；docker-compose(pg+api+web)
- SQLAlchemy 16 MVP 表 + Alembic 初版迁移(无漂移，users stub)
- FastAPI 骨架：统一错误信封(带 request_id) + structlog + /jobs/:id + OpenAPI
- Next.js 骨架：纸感主题 token + OpenAPI→TS 客户端代码生成(gen:api)
- CI(ruff/mypy/pytest + pg service + alembic 漂移校验)
- 四份设计规格(PRODUCT/UX/ARCHITECTURE/DEV_PLAN) + CLAUDE.md

apps/web/components/AppShell.tsx

@@ -0,0 +1,46 @@
+import Link from "next/link";
+import type { ReactNode } from "react";
+
+import { LeftNav } from "./LeftNav";
+
+interface AppShellProps {
+  children: ReactNode;
+  // 顶栏右侧的上下文信息（如作品名 / 进度）。
+  title?: string;
+  subtitle?: string;
+}
+
+// 全局外壳：顶栏(64px) + 左导航 + 主区（UX §5.1）。
+export function AppShell({ children, title, subtitle }: AppShellProps) {
+  return (
+    <div className="min-h-screen">
+      <header className="flex h-16 items-center gap-4 border-b border-line bg-panel px-6">
+        <Link
+          href="/"
+          className="font-serif text-xl text-cinnabar"
+          aria-label="返回作品库"
+        >
+          墨痕
+        </Link>
+        {title ? (
+          <span className="font-serif text-lg text-ink">{title}</span>
+        ) : null}
+        {subtitle ? (
+          <span className="font-mono text-xs text-ink-soft">{subtitle}</span>
+        ) : null}
+        <nav className="ml-auto flex items-center gap-4 text-sm">
+          <Link
+            href="/settings/providers"
+            className="text-ink-soft hover:text-cinnabar"
+          >
+            设置
+          </Link>
+        </nav>
+      </header>
+      <div className="flex">
+        <LeftNav />
+        <main className="min-h-[calc(100vh-4rem)] flex-1 bg-bg">{children}</main>
+      </div>
+    </div>
+  );
+}