feat(frontend): v2 rewrite — Vite + React + TS readonly SPA

Replaces the v1 CRA app (which targeted the obsolete Python Dialectic
backend) with a fresh Vite + React 18 + TypeScript scaffold that talks
to Dialectic.Backend Go v2.

Pages (all readonly — propose/signup/post are agent-only by design):
  - /                       TopicList — filter by status, paginated
  - /topics/:id             TopicDetail — meta + camps + transcript
                            (polling every 8s)
  - /topics/:id/verdict     Verdict permalink (shareable)
  - /agents/:id             AgentActivity — admin diagnostics card

Stack:
  - Vite 5 + React 18 + react-router-dom 6
  - Pure ESM, NodeNext-style imports, .tsx
  - Style: ~/STYLE.md tokens (IBM Plex Mono + Major Mono Display +
    --acid #d8ff3e on --ink #080a0d, with subtle blueprint grid wash)

Auth:
  - v1 dev-bypass only — VITE_OIDC_DEV_BYPASS auto-attaches
    x-dev-bypass header. Real Keycloak OIDC redirect ships as v2.
  - Admin endpoints (x-dialectic-admin-key) prompt on first visit
    and store key in localStorage. Never baked into bundle. Never
    sent to non-admin endpoints.

Backend pairing:
  - Dialectic.Backend@0b16b52 adds GET /api/admin/agents/{id} for the
    AgentActivity page. AgentActivity calls it via the admin-key
    branch in api.ts.

Deploy:
  - Multi-stage Dockerfile (node:22-alpine build → nginx:1.27-alpine
    serve). nginx.conf reverse-proxies /api/ → dialectic-backend:8090
    so the browser sees one origin (no CORS).

Reuses the existing hzhang/Dialectic.Frontend repo — old CRA contents
nuked in this commit. History preserved on master.

src/App.tsx

@@ -0,0 +1,54 @@
+import { NavLink, Route, Routes } from 'react-router-dom';
+import { useAuth } from './auth';
+import { TopicListPage } from './pages/TopicList';
+import { TopicDetailPage } from './pages/TopicDetail';
+import { VerdictPage } from './pages/Verdict';
+import { AgentActivityPage } from './pages/AgentActivity';
+import { NotFoundPage } from './pages/NotFound';
+import './styles/app.css';
+
+export function App() {
+  const { user, devBypass } = useAuth();
+  return (
+    <div className="app">
+      <header className="app-header">
+        <div className="app-header-inner">
+          <a href="/" className="app-brand brand-d">
+            dialectic
+          </a>
+          <nav className="app-nav">
+            <NavLink to="/" end>
+              topics
+            </NavLink>
+            <NavLink to="/agents/recruiter">agents</NavLink>
+          </nav>
+          <div className="app-user">
+            {user ? (
+              <span className="eyebrow">{user.label}</span>
+            ) : (
+              <span className="eyebrow">readonly</span>
+            )}
+          </div>
+        </div>
+        {devBypass && (
+          <div className="app-dev-banner">
+            DEV BYPASS ACTIVE — auto-attaching <code>x-dev-bypass</code> on
+            every request; do not run this build in production
+          </div>
+        )}
+      </header>
+      <main className="app-main">
+        <Routes>
+          <Route path="/" element={<TopicListPage />} />
+          <Route path="/topics/:id" element={<TopicDetailPage />} />
+          <Route path="/topics/:id/verdict" element={<VerdictPage />} />
+          <Route path="/agents/:id" element={<AgentActivityPage />} />
+          <Route path="*" element={<NotFoundPage />} />
+        </Routes>
+      </main>
+      <footer className="app-footer">
+        <span className="eyebrow">hangman lab · dialectic v2</span>
+      </footer>
+    </div>
+  );
+}