fix(sidebar): stop counts flickering to 0 and layout collapsing during nav transitions

[eliran-ops]

Summary

SKY-824 — two related Resources sidebar regressions:

• 39 — Sidebar resource counts go to "0" for every kind during initial load instead of preserving last-known values or hiding counts. Looks like ~2s of "data loss".
• 65 — The hierarchical sidebar (Workloads / Networking / …) briefly collapses to the flat fallback (Pods / Deployments / etc., all counts 0) during nav transitions, causing UI flicker.

Root cause

• The count map coerced every missing key to 0 (`resourceCounts[key] ?? 0`), so during the window between "nothing fetched yet" and "counts payload arrived" every kind rendered a confident "0" badge.
• During route transitions the parent briefly re-renders with `apiResources={undefined}` for one frame; the sidebar then drops to its flat `CORE_RESOURCE_TYPES` fallback for that frame and snaps back to hierarchical the next.

Fix

1. New shared hook `useLastDefined(value)` in `packages/k8s-ui/src/hooks/`. Latches the last non-undefined value via a ref so it doesn't trigger re-renders, and never replaces a defined value with stale data. 5 unit tests pin the contract.
2. `ResourcesSidebar` wraps both `apiResources` and `resourceCounts` in `useLastDefined` so a one-frame undefined drop doesn't collapse the layout or blank the badges.
3. Distinguish loading from zero in counts:
   • Internal `counts: Record<string, number | null>`. `null` = not loaded; `0` = API confirmed zero.
   • Badge UI renders an en-dash (`–`) for null with `text-theme-text-disabled` so loading badges are visually subordinate.
   • Category "show this kind" filter treats null as "show, might have data" rather than hiding it.
   • Category totals coerce nulls to 0 for the collapsed-summary badge — honest because the total is over a known finite set of kinds.

Out of scope

The `Checks` page mentioned in bug 40 lives in `radar-hub-web`, not radar OSS.

Test plan

• `cd packages/k8s-ui && npm test` — all passing (5 new for `useLastDefined`)
• `cd web && npm run tsc` — clean
• `cd web && npm run build` — clean

Made with Cursor

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Note

Low Risk
UI-only behavior change plus a small shared hook; main risk is subtle regressions in sidebar visibility/count rendering during loading states.

Overview
Prevents ResourcesSidebar from flickering during navigation/loading by latching the last non-undefined apiResources and resourceCounts values via a new useLastDefined hook.

Updates count handling to distinguish loading vs zero (number | null), rendering a placeholder dash and keeping kinds visible while counts are unknown, instead of briefly showing 0 everywhere. Adds unit tests for useLastDefined and exports it from the hooks index.

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