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/App.tsx

@@ -0,0 +1,62 @@
+import { Route, Switch, Link, useLocation } from 'wouter';
+import { CommandCenter } from './pages/CommandCenter';
+import { NodeDetail } from './pages/NodeDetail';
+import { StorageBrowser } from './pages/StorageBrowser';
+import { RefTrackingPage } from './pages/RefTrackingPage';
+
+export default function App() {
+  const [loc] = useLocation();
+  const tab = (path: string, label: string) => {
+    const active = loc === path || (path !== '/' && loc.startsWith(path));
+    return (
+      <Link href={path}>
+        <a
+          className={[
+            'px-3 py-2 text-sm border-b-2 transition-colors',
+            active
+              ? 'border-emerald-400 text-emerald-300'
+              : 'border-transparent text-slate-400 hover:text-slate-100',
+          ].join(' ')}
+        >
+          {label}
+        </a>
+      </Link>
+    );
+  };
+  return (
+    <div className="min-h-screen">
+      <header className="border-b border-slate-800 bg-slate-950/80 backdrop-blur sticky top-0 z-10">
+        <div className="max-w-7xl mx-auto px-4 flex items-center gap-6">
+          <Link href="/">
+            <a className="font-mono text-lg text-emerald-300 py-3">
+              clawstor · command center
+            </a>
+          </Link>
+          <nav className="flex">
+            {tab('/', 'Overview')}
+            {tab('/storage/blobs', 'Blobs')}
+            {tab('/storage/tags', 'Tags')}
+            {tab('/storage/refs', 'Refs')}
+            {tab('/storage/snapshots', 'Snapshots')}
+            {tab('/refs/tracking', 'Ref-tracking')}
+          </nav>
+        </div>
+      </header>
+      <main className="max-w-7xl mx-auto p-4">
+        <Switch>
+          <Route path="/" component={CommandCenter} />
+          <Route path="/nodes/:name">
+            {(params) => <NodeDetail name={params.name} />}
+          </Route>
+          <Route path="/storage/:tab">
+            {(params) => <StorageBrowser tab={params.tab as any} />}
+          </Route>
+          <Route path="/refs/tracking" component={RefTrackingPage} />
+          <Route>
+            <div className="text-slate-400 py-12 text-center">404</div>
+          </Route>
+        </Switch>
+      </main>
+    </div>
+  );
+}