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.

Dockerfile

@@ -1,8 +1,26 @@
-FROM node:21-alpine
-RUN apk add --no-cache bash
+# Multi-stage build:
+#   1. node:alpine — install + vite build → dist/
+#   2. nginx:alpine — serve dist/ + reverse-proxy /api/ to the backend
+#
+# Build args:
+#   VITE_DIALECTIC_API_BASE — defaults to '/api' (same-origin via nginx)
+#   VITE_OIDC_DEV_BYPASS    — set to a token string to bake dev-bypass
+#                             into the bundle (DO NOT set in prod images)
+
+FROM node:22-alpine AS build
 WORKDIR /app
-COPY package*.json ./
-RUN npm install
+COPY package.json package-lock.json* ./
+RUN npm ci --no-audit --no-fund
 COPY . .
-EXPOSE 3000
-CMD ["npm", "start"]
+ARG VITE_DIALECTIC_API_BASE=/api
+ARG VITE_OIDC_DEV_BYPASS=
+ENV VITE_DIALECTIC_API_BASE=$VITE_DIALECTIC_API_BASE
+ENV VITE_OIDC_DEV_BYPASS=$VITE_OIDC_DEV_BYPASS
+RUN npm run build
+
+FROM nginx:1.27-alpine AS serve
+COPY --from=build /app/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s --retries=3 \
+  CMD wget -qO- http://127.0.0.1/index.html >/dev/null || exit 1