refactor(frontend): extract NodeCard and OverviewTab from FleetView (F5-3+F5-5) (#919)

* refactor(frontend): extract NodeCard and OverviewTab from FleetView (F5-3+F5-5)

Folds F5-3 (NodeCard) and F5-5 (OverviewTab) into a single PR since
NodeCard was never previously extracted.

- Move FleetNodeStats, FleetNodeSystemStats, FleetNode into FleetView/types.ts
- Extract NodeCard (~200 LOC) including UsageBar, ContainerRow, StackSection
  sub-components and getNodeCpu/getNodeMem/getNodeDisk/isCritical helpers
- Extract OverviewTab (~115 LOC); delegates to NodeCard, OverviewToolbar,
  FleetTopology; receives all state as flat props from FleetView
- FleetView.tsx drops from ~1,107 to ~480 LOC (overview inline body gone)
- No logic moved; all state, hooks, and computed values remain in FleetView.tsx
- formatBytes consolidated to @/lib/utils; node helpers exported from NodeCard
- allNodes wrapped in useMemo to prevent unnecessary child re-renders

* fix(frontend): move node utility functions to nodeUtils.ts to fix react-refresh lint error

getNodeCpu, getNodeMem, getNodeDisk, and isCritical were exported from NodeCard.tsx
alongside a React component, violating the react-refresh/only-export-components rule.
Moving them to a dedicated nodeUtils.ts resolves the ESLint error without changing any logic.

frontend/src/components/FleetView/nodeUtils.ts

@@ -0,0 +1,17 @@
+import type { FleetNode } from './types';
+
+export function getNodeCpu(node: FleetNode): number {
+    return node.systemStats ? parseFloat(node.systemStats.cpu.usage) : 0;
+}
+
+export function getNodeMem(node: FleetNode): number {
+    return node.systemStats ? parseFloat(node.systemStats.memory.usagePercent) : 0;
+}
+
+export function getNodeDisk(node: FleetNode): number {
+    return node.systemStats?.disk ? parseFloat(node.systemStats.disk.usagePercent) : 0;
+}
+
+export function isCritical(node: FleetNode): boolean {
+    return getNodeCpu(node) > 90 || getNodeDisk(node) > 90;
+}