feat(fleet): interactive topology with ReactFlow hub-and-spoke layout (#713)

Replace the static SVG fleet topology with a ReactFlow canvas laid out
via dagre. Each node renders as a rack card with status pill, type
badge, CPU/MEM/DISK bars, and stack/running counts. Pan, zoom, drag,
and minimap are enabled; user-dragged positions persist across the
30-second poll so live metric updates no longer reset layout.

frontend/src/components/FleetView.tsx

@@ -993,6 +993,9 @@ export function FleetView({ onNavigateToNode }: FleetViewProps) {
         status: n.status,
         cpuPercent: getNodeCpu(n),
         memPercent: getNodeMem(n),
+        diskPercent: getNodeDisk(n),
+        stackCount: n.stacks?.length ?? 0,
+        runningCount: n.stats?.active ?? 0,
         critical: n.status === 'online' && isCritical(n),
     })), [processedNodes]);