Fix Thresholds page crash on guest filesystems with omitted usage

The unified resources payload serializes guest filesystem numerics with
omitempty, so a disk whose usage is exactly zero arrives with the field
absent. The guest filesystem projection guarded with usage < 0, which
undefined passes, then dereferenced usage.toFixed and took down the whole
Alerts > Thresholds page behind the error boundary (#1663).

Treat absent numerics as zero and derive the percentage from used/total
when the serialized value is missing; a negative usage still means the
poller could not read the filesystem and stays excluded.

frontend-modern/src/features/alerts/thresholds/hooks/__tests__/useThresholdsGuestData.test.ts

@@ -72,6 +72,69 @@ describe('useThresholdsGuestData guest filesystems', () => {
     });
   });
 
+  it('projects filesystems whose serialized usage was omitted without crashing', () => {
+    createRoot((dispose) => {
+      const props = {
+        allGuests: () => [
+          {
+            id: 'cluster-a:node-1:101',
+            name: 'files',
+            platformId: 'cluster-a',
+            status: 'online',
+            type: 'ct',
+            lastSeen: 1,
+            proxmox: {
+              instance: 'cluster-a',
+              node: 'node-1',
+              vmid: 101,
+              // The unified resources payload drops zero-valued numerics, so
+              // an untouched filesystem arrives with no usage/used fields.
+              disks: [
+                {
+                  device: 'rootfs',
+                  mountpoint: '/',
+                  total: 107374182400,
+                  type: 'ext4',
+                },
+                {
+                  device: 'mp0',
+                  mountpoint: '/data',
+                  total: 214748364800,
+                  used: 107374182400,
+                  free: 107374182400,
+                  type: 'ext4',
+                },
+              ],
+            },
+          },
+        ],
+        backupDefaults: () => ({ enabled: false }),
+        guestDefaults: { disk: 85 },
+        nodes: [],
+        overrides: () => [],
+        snapshotDefaults: () => ({ enabled: false }),
+      } as unknown as ThresholdsTableProps;
+
+      const data = useThresholdsGuestData({
+        props,
+        editingId: () => null,
+        searchTerm: () => '',
+      });
+
+      expect(data.guestDisksWithOverrides()).toEqual([
+        expect.objectContaining({
+          name: '/',
+          subtitle: '0.0 / 100.0 GB · 0.0%',
+        }),
+        expect.objectContaining({
+          name: '/data',
+          subtitle: '100.0 / 200.0 GB · 50.0%',
+        }),
+      ]);
+      dispose();
+    });
+  });
+
   it('keeps a persisted filesystem override visible after its guest disappears', () => {
     createRoot((dispose) => {
       const props = {

frontend-modern/src/features/alerts/thresholds/hooks/useThresholdsGuestData.ts

@@ -117,7 +117,16 @@ export function useThresholdsGuestData(inputs: ThresholdsDataInputs) {
       const guestIdentity = getGuestOverrideIdentity(guest);
 
       disks.forEach((disk: Disk) => {
-        if (!disk || disk.total <= 0 || disk.usage < 0) return;
+        // The unified resources payload omits zero-valued numerics, so
+        // total/used/usage may all be absent; usage < 0 is the poller's
+        // "unknown" sentinel and stays excluded.
+        const total = disk?.total ?? 0;
+        const used = disk?.used ?? 0;
+        if (!disk || total <= 0 || (typeof disk.usage === 'number' && disk.usage < 0)) return;
+        const usagePercent =
+          typeof disk.usage === 'number' && Number.isFinite(disk.usage)
+            ? disk.usage
+            : (used / total) * 100;
 
         const candidates = guestDiskOverrideIdCandidates(guest, disk.mountpoint, disk.device);
         const storageId = guestDiskOverrideStorageId(guest, disk.mountpoint, disk.device);
@@ -153,7 +162,7 @@ export function useThresholdsGuestData(inputs: ThresholdsDataInputs) {
           disabled: override?.disabled || false,
           thresholds: override?.thresholds || {},
           defaults: { disk: props.guestDefaults.disk },
-          subtitle: `${(disk.used / 1024 / 1024 / 1024).toFixed(1)} / ${(disk.total / 1024 / 1024 / 1024).toFixed(1)} GB · ${disk.usage.toFixed(1)}%`,
+          subtitle: `${(used / 1024 / 1024 / 1024).toFixed(1)} / ${(total / 1024 / 1024 / 1024).toFixed(1)} GB · ${usagePercent.toFixed(1)}%`,
         } satisfies TableResource);
       });
     });