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/pages/AgentActivity.tsx

@@ -0,0 +1,132 @@
+import { useEffect, useState } from 'react';
+import { useParams } from 'react-router-dom';
+import { getAgentSummary, HttpError } from '../api';
+import type { AgentSummary } from '../types';
+import { Link } from 'react-router-dom';
+import { fmtRelative, fmtTime } from '../util';
+
+export function AgentActivityPage() {
+  const { id } = useParams<{ id: string }>();
+  const [data, setData] = useState<AgentSummary | null>(null);
+  const [err, setErr] = useState<string | null>(null);
+  const [needsKey, setNeedsKey] = useState(false);
+  const [pendingKey, setPendingKey] = useState('');
+
+  useEffect(() => {
+    if (!id) return;
+    const ac = new AbortController();
+    getAgentSummary(id, ac.signal)
+      .then((d) => {
+        setData(d);
+        setErr(null);
+        setNeedsKey(false);
+      })
+      .catch((e) => {
+        if ((e as Error).name === 'AbortError') return;
+        if (e instanceof HttpError && (e.status === 401 || e.status === 403)) {
+          setNeedsKey(true);
+          setErr(null);
+        } else {
+          setErr((e as Error).message);
+        }
+      });
+    return () => ac.abort();
+  }, [id]);
+
+  function saveKey() {
+    if (!pendingKey.trim()) return;
+    localStorage.setItem('dialectic-admin-key', pendingKey.trim());
+    setPendingKey('');
+    window.location.reload();
+  }
+
+  if (!id) return <div className="err">missing agent id</div>;
+
+  return (
+    <div>
+      <span className="eyebrow">agent</span>
+      <h1>{id}</h1>
+
+      {needsKey && (
+        <div className="aa-admin-prompt">
+          <div>
+            this page calls an <strong>admin endpoint</strong> — paste your
+            <code> x-dialectic-admin-key</code> (stored in localStorage; never
+            sent except to <code>/api/admin/*</code>).
+          </div>
+          <input
+            type="password"
+            value={pendingKey}
+            onChange={(e) => setPendingKey(e.target.value)}
+            placeholder="admin key…"
+          />
+          <div className="row" style={{ marginTop: 8 }}>
+            <button onClick={saveKey}>save + reload</button>
+            <span className="muted">key never leaves this browser</span>
+          </div>
+        </div>
+      )}
+
+      {err && <div className="err">{err}</div>}
+
+      {data && (
+        <>
+          <div className="aa-summary-grid">
+            <div className="aa-card">
+              <div className="aa-card-value">
+                {data.key_provisioned ? 'yes' : 'no'}
+              </div>
+              <div className="aa-card-label">key provisioned</div>
+            </div>
+            <div className="aa-card">
+              <div className="aa-card-value">{data.signups_count}</div>
+              <div className="aa-card-label">signups</div>
+            </div>
+            <div className="aa-card">
+              <div className="aa-card-value">{data.arguments_count}</div>
+              <div className="aa-card-label">arguments</div>
+            </div>
+            <div className="aa-card">
+              <div className="aa-card-value">{data.verdicts_count}</div>
+              <div className="aa-card-label">verdicts</div>
+            </div>
+          </div>
+
+          <h2>recent topics</h2>
+          {data.recent_topics.length === 0 ? (
+            <div className="td-empty">no recent topics for this agent</div>
+          ) : (
+            <table className="tl-table">
+              <thead>
+                <tr>
+                  <th>topic</th>
+                  <th>role</th>
+                  <th>status</th>
+                  <th>last action</th>
+                </tr>
+              </thead>
+              <tbody>
+                {data.recent_topics.map((t) => (
+                  <tr key={t.topic_id}>
+                    <td>
+                      <Link to={`/topics/${t.topic_id}`}>{t.title}</Link>
+                    </td>
+                    <td>
+                      <span className={`camp-${t.role}`}>{t.role}</span>
+                    </td>
+                    <td>
+                      <span className={`pill pill-${t.status}`}>{t.status}</span>
+                    </td>
+                    <td title={fmtTime(t.last_action_at)}>
+                      {fmtRelative(t.last_action_at)}
+                    </td>
+                  </tr>
+                ))}
+              </tbody>
+            </table>
+          )}
+        </>
+      )}
+    </div>
+  );
+}