dashboard-v2 PR 2: frontend SPA + serve integration

React 19 + Vite + Tailwind + wouter (tiny router, no external
state library). Consumes the /api/v2/* endpoints shipped in PR 1.
Serves under /v2/* so the legacy dashboard at / stays live.

Pages:
* CommandCenter (/)      — fleet strip + this-node stat tiles
* NodeDetail (/nodes/:name) — per-node deep dive
* StorageBrowser (/storage/{blobs,tags,refs,snapshots}) — tables
  with prefix filter
* RefTrackingPage (/refs/tracking) — grouped by repo

Backend changes:
* claw-store serve grows --v2-static-dir <path>
* build_app split into build_app_with_v2 for the extra static
  mount
* /v2/* falls through to index.html so wouter client routing works

New systemd unit: clawstor-dashboard.service. Points at both
static dirs; installs on any node.

dashboard/ (legacy) untouched. dashboard-v2/ built to
target/dashboard-v2/dist for deploy.

Deploy sequence per node:
1. cp target/release/claw-store  ~/clawstor-deploy/
2. rsync dashboard-v2/dist/      ~/clawstor-deploy/dashboard-v2/
3. cp deploy/systemd/clawstor-dashboard.service ~/.config/systemd/user/
4. systemctl --user daemon-reload && enable --now clawstor-dashboard.service

Cross-node fan-out for /api/v2/node/:name/status is PR 3.
Action POSTs (scrub/gc/snapshot/pin) are PR 4.

dashboard-v2/src/pages/RefTrackingPage.tsx

@@ -0,0 +1,95 @@
+import { useEffect, useMemo, useState } from 'react';
+import { api, RefTrackingItem, fmtAge } from '../lib/api';
+
+export function RefTrackingPage() {
+  const [rows, setRows] = useState<RefTrackingItem[]>([]);
+  const [repoFilter, setRepoFilter] = useState('');
+  const [err, setErr] = useState<string | null>(null);
+
+  useEffect(() => {
+    api
+      .refTracking(repoFilter)
+      .then(setRows)
+      .catch((e) => setErr(String(e)));
+  }, [repoFilter]);
+
+  // Group by repo for a cleaner display.
+  const grouped = useMemo(() => {
+    const g = new Map<string, RefTrackingItem[]>();
+    for (const r of rows) {
+      const arr = g.get(r.repo) ?? [];
+      arr.push(r);
+      g.set(r.repo, arr);
+    }
+    return [...g.entries()].sort((a, b) => a[0].localeCompare(b[0]));
+  }, [rows]);
+
+  return (
+    <div className="space-y-4">
+      <div>
+        <h1 className="text-2xl font-semibold text-slate-100">Ref-tracking</h1>
+        <p className="text-sm text-slate-500 mt-1">
+          Every cached fingerprint's producing <span className="font-mono">(repo, git-ref)</span>.
+          Feeds the nightly <span className="font-mono">cluster-ref-sweep</span> that reaps
+          fingerprints whose refs are gone from Gitea.
+        </p>
+      </div>
+
+      <input
+        value={repoFilter}
+        onChange={(e) => setRepoFilter(e.target.value)}
+        placeholder="filter repo — e.g. clawverse/clawstor"
+        className="w-full sm:w-96 rounded bg-slate-900 border border-slate-800 px-3 py-2 text-sm font-mono"
+      />
+
+      {err && (
+        <div className="rounded border border-red-800 bg-red-950/40 p-3 text-red-300 text-sm">
+          {err}
+        </div>
+      )}
+
+      {grouped.length === 0 && (
+        <div className="text-slate-500 text-sm py-6">no ref-tracking entries yet</div>
+      )}
+
+      {grouped.map(([repo, items]) => (
+        <div key={repo} className="rounded border border-slate-800 overflow-hidden">
+          <div className="bg-slate-900/60 px-4 py-2 flex items-baseline justify-between">
+            <span className="font-mono text-sm text-emerald-300">{repo}</span>
+            <span className="text-xs text-slate-500">
+              {items.length} fingerprint{items.length === 1 ? '' : 's'}
+            </span>
+          </div>
+          <table className="w-full text-sm">
+            <thead className="text-slate-500 uppercase text-xs tracking-wider">
+              <tr>
+                <th className="text-left px-4 py-2 font-normal">fingerprint</th>
+                <th className="text-left px-4 py-2 font-normal">refs</th>
+                <th className="text-left px-4 py-2 font-normal">first seen</th>
+                <th className="text-left px-4 py-2 font-normal">last seen</th>
+              </tr>
+            </thead>
+            <tbody>
+              {items.map((it) => (
+                <tr key={it.fingerprint_hex} className="border-t border-slate-900">
+                  <td className="px-4 py-2 font-mono text-xs">
+                    {it.fingerprint_hex.slice(0, 24)}…
+                  </td>
+                  <td className="px-4 py-2 font-mono text-xs">
+                    {it.refs.join(', ')}
+                  </td>
+                  <td className="px-4 py-2 text-slate-400">
+                    {fmtAge(it.first_seen_unix)}
+                  </td>
+                  <td className="px-4 py-2 text-slate-400">
+                    {fmtAge(it.last_seen_unix)}
+                  </td>
+                </tr>
+              ))}
+            </tbody>
+          </table>
+        </div>
+      ))}
+    </div>
+  );
+}