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/lib/api.ts

@@ -1,6 +1,38 @@
 // Thin fetch wrappers around /api/v2/*. Kept minimal — no state
 // library; each page owns its own useEffect + useState.
 
+export interface FilesystemUsage {
+  mount_point: string;
+  total_bytes: number;
+  available_bytes: number;
+  used_bytes: number;
+}
+
+export interface HotTierUsage {
+  used_bytes: number;
+  max_bytes: number;
+  pinned_bytes: number | null;
+}
+
+export interface MountStatus {
+  path: string;
+  active: boolean;
+}
+
+export interface CacheSummary {
+  hits: number;
+  misses: number;
+  bytes_served: number;
+  bytes_ingested: number;
+  hit_rate: number;
+}
+
+export interface TimerStatus {
+  unit: string;
+  next_fire_unix: number | null;
+  last_result: string | null;
+}
+
 export interface NodeStatusV2 {
   node_name: string;
   zone: string;
@@ -12,6 +44,11 @@ export interface NodeStatusV2 {
   ref_tracking_count: number;
   blob_store_bytes: number;
   rustc_release: string | null;
+  filesystem: FilesystemUsage | null;
+  hot: HotTierUsage | null;
+  mount: MountStatus | null;
+  cache: CacheSummary | null;
+  timers: TimerStatus[];
   online: boolean;
   error: string | null;
 }