FleetHealth PR 2 (frontend): human-oriented landing + advanced menu

Reworks the SPA around the new DashboardStatus fields. The
landing is now a fleet-health dashboard aimed at a layperson —
disk gauges, mount ✓/✗, cache hit rate, next scheduled job.
No hex, no primitives.

Nav restructure:
* Primary: 'Fleet health' (just the landing).
* 'View Advanced ▾' dropdown reveals: Blobs / Tags / Refs /
  Snapshots / Ref-tracking. Routes moved under /advanced/*.

New components:
* StorageBar — horizontal used/total bar with pinned/evictable
  split, health-color threshold at 60/85%.
* NodeCard — traffic-light dot + disk + hot tier bars + mount
  state + cache hit rate + next-timer countdown. Whole card
  is a link into node detail.

New CommandCenter:
* Fleet-wide storage roll-up card (sum of every node's disk).
* Grid of NodeCards.
* 10s poll cadence retained from previous version.

Existing StorageBrowser + RefTrackingPage moved behind
/advanced/* routes; internal component code untouched.

dashboard-v2/src/components/StorageBar.tsx

@@ -0,0 +1,57 @@
+import { fmtBytes } from '../lib/api';
+
+interface Props {
+  label: string;
+  used: number;
+  total: number;
+  // Optional split: a portion of `used` that's "pinned" (won't be
+  // evicted). Rendered green; the rest of used is amber.
+  pinned?: number | null;
+}
+
+/// Big horizontal storage bar. Read-only. Renders green (pinned,
+/// safe) + amber (used, evictable) + slate (free). Health color
+/// on the label based on fill %.
+export function StorageBar({ label, used, total, pinned }: Props) {
+  const safeTotal = Math.max(total, 1);
+  const pct = Math.min(100, Math.round((used / safeTotal) * 100));
+  const pinnedPct = pinned
+    ? Math.min(100, Math.round((pinned / safeTotal) * 100))
+    : 0;
+  const evictablePct = Math.max(0, pct - pinnedPct);
+  const bar = pct < 60 ? 'ok' : pct < 85 ? 'warn' : 'err';
+  const barText = {
+    ok: 'text-emerald-300',
+    warn: 'text-amber-300',
+    err: 'text-red-300',
+  }[bar];
+
+  return (
+    <div>
+      <div className="flex items-baseline justify-between text-xs mb-1">
+        <span className="text-slate-500 uppercase tracking-wider">{label}</span>
+        <span className={`font-mono ${barText}`}>
+          {fmtBytes(used)} / {fmtBytes(total)} · {pct}%
+        </span>
+      </div>
+      <div className="h-2.5 w-full rounded-full bg-slate-800 overflow-hidden flex">
+        {pinnedPct > 0 && (
+          <div
+            className="bg-emerald-500 h-full"
+            style={{ width: `${pinnedPct}%` }}
+            title={`Pinned: ${fmtBytes(pinned!)}`}
+          />
+        )}
+        {evictablePct > 0 && (
+          <div
+            className={`h-full ${
+              bar === 'err' ? 'bg-red-500' : bar === 'warn' ? 'bg-amber-500' : 'bg-emerald-600'
+            }`}
+            style={{ width: `${evictablePct}%` }}
+            title={`Used: ${fmtBytes(used - (pinned ?? 0))}`}
+          />
+        )}
+      </div>
+    </div>
+  );
+}