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/auth.tsx

@@ -0,0 +1,33 @@
+// AuthProvider — minimal v1: dev-bypass only, no OIDC redirect yet.
+//
+// Real OIDC + Keycloak ships when the backend's OIDC middleware is
+// fully wired (Phase 4 in DIALECTIC-V2-DESIGN.md). Until then this
+// just surfaces whether dev-bypass is on so the UI can show a banner
+// ("dev mode — running as operator") and route admin-gated pages.
+
+import { createContext, useContext, type ReactNode } from 'react';
+
+interface AuthCtx {
+  // Whether dev-bypass token is configured at build time.
+  devBypass: boolean;
+  // Display label for the "current user" — in dev-bypass mode this is
+  // the literal env value (a placeholder until real OIDC).
+  user: { id: string; label: string } | null;
+}
+
+const Ctx = createContext<AuthCtx>({
+  devBypass: false,
+  user: null,
+});
+
+export function AuthProvider({ children }: { children: ReactNode }) {
+  const devBypass = Boolean(import.meta.env.VITE_OIDC_DEV_BYPASS);
+  const user = devBypass
+    ? { id: 'dev-operator', label: 'dev-operator (bypass)' }
+    : null;
+  return <Ctx.Provider value={{ devBypass, user }}>{children}</Ctx.Provider>;
+}
+
+export function useAuth(): AuthCtx {
+  return useContext(Ctx);
+}