feat: complete phase 5 -- error hardening, frontend, Docker, demo, docs

Backend:
- ConversationTracker: Protocol + PostgresConversationTracker for lifecycle tracking
- Error handler: ErrorCategory enum, classify_error(), with_retry() exponential backoff
- Wire PostgresAnalyticsRecorder + ConversationTracker into ws_handler
- Rate limiting (10 msg/10s per thread), edge case hardening
- Health endpoint GET /api/health, version 0.5.0
- Demo seed data script + sample OpenAPI spec

Frontend (all new):
- React Router with NavBar (Chat / Replay / Dashboard / Review)
- ReplayListPage + ReplayPage with ReplayTimeline component
- DashboardPage with MetricCard, range selector, zero-state
- ReviewPage for OpenAPI classification review
- ErrorBanner for WebSocket disconnect handling
- API client (api.ts) with typed fetch wrappers

Infrastructure:
- Frontend Dockerfile (multi-stage node -> nginx)
- nginx.conf with SPA routing + API/WS proxy
- docker-compose.yml with frontend service + healthchecks
- .env.example files (root + backend)

Documentation:
- README.md with quick start and architecture
- Agent configuration guide
- OpenAPI import guide
- Deployment guide
- Demo script

48 new tests, 449 total passing, 92.87% coverage

frontend/src/components/NavBar.tsx

@@ -0,0 +1,64 @@
+import { NavLink } from "react-router-dom";
+
+const navLinks = [
+  { to: "/", label: "Chat", exact: true },
+  { to: "/replay", label: "Replay" },
+  { to: "/dashboard", label: "Dashboard" },
+  { to: "/review", label: "API Review" },
+];
+
+const styles: Record<string, React.CSSProperties> = {
+  nav: {
+    display: "flex",
+    alignItems: "center",
+    gap: "0",
+    padding: "0 16px",
+    borderBottom: "1px solid #e0e0e0",
+    background: "#fff",
+    height: "48px",
+    boxShadow: "0 1px 4px rgba(0,0,0,0.06)",
+  },
+  brand: {
+    fontWeight: 700,
+    fontSize: "16px",
+    color: "#1a1a1a",
+    marginRight: "24px",
+    textDecoration: "none",
+  },
+  link: {
+    padding: "0 14px",
+    height: "48px",
+    display: "flex",
+    alignItems: "center",
+    fontSize: "14px",
+    color: "#555",
+    textDecoration: "none",
+    borderBottom: "2px solid transparent",
+    transition: "color 0.15s, border-color 0.15s",
+  },
+  activeLink: {
+    color: "#1976d2",
+    borderBottom: "2px solid #1976d2",
+  },
+};
+
+export function NavBar() {
+  return (
+    <nav style={styles.nav}>
+      <span style={styles.brand}>Smart Support</span>
+      {navLinks.map(({ to, label }) => (
+        <NavLink
+          key={to}
+          to={to}
+          end={to === "/"}
+          style={({ isActive }) => ({
+            ...styles.link,
+            ...(isActive ? styles.activeLink : {}),
+          })}
+        >
+          {label}
+        </NavLink>
+      ))}
+    </nav>
+  );
+}