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.

claw-store/src/main.rs

@@ -64,8 +64,13 @@ enum Cmd {
     Serve {
         #[arg(long, default_value = "7700")]
         port: u16,
+        /// Legacy dashboard static assets (served under `/`).
         #[arg(long)]
         static_dir: Option<PathBuf>,
+        /// dashboard-v2 static assets (served under `/v2/*`).
+        /// See docs/dashboard-v2.md.
+        #[arg(long)]
+        v2_static_dir: Option<PathBuf>,
     },
     /// Mark a project as pinned — survives every GC pass (stale + LRU)
     Pin { project: String },
@@ -350,8 +355,8 @@ async fn main() -> Result<()> {
         Cmd::ListSnapshots { project } => cmd_list_snapshots(&cfg, &zfs, &project)?,
         Cmd::Restore { project, snapshot } => cmd_restore(&cfg, &zfs, &project, &snapshot)?,
         Cmd::Replicate => cmd_replicate(&cfg, &zfs)?,
-        Cmd::Serve { port, static_dir } =>
-            serve::run_server(cfg, manifest, port, static_dir).await?,
+        Cmd::Serve { port, static_dir, v2_static_dir } =>
+            serve::run_server(cfg, manifest, port, static_dir, v2_static_dir).await?,
         Cmd::Pin { project } => cmd_set_pin(&manifest_path, &project, true)?,
         Cmd::Unpin { project } => cmd_set_pin(&manifest_path, &project, false)?,
         Cmd::ClusterProbe { peer } => cmd_cluster_probe(&cfg, &peer).await?,