v0.40.3 — fix IntervalHeatmap null-payload crash (v0.40.2 regression)

Hotfix release. Fixes a client-side crash introduced in v0.40.2 (#150) and restores the interval-heatmap widget.

• #166 — IntervalHeatmap dereferenced payload!.grid in eagerly-evaluated children during the loading tick (payload null), throwing an unguarded render-time TypeError that took down the whole dashboard whenever the heatmap widget was mounted. Now gated on {payload && (…)}. Siblings (LoadShape/History) audited safe.

Verified on staging against the exact crash repro (default layout mounts the heatmap → renders skeleton/data, 0 console errors; previously "Application error"). Docker test 67 files / 973 tests; production build green. No schema change.

Prod was rolled back to v0.40.1 during the incident; this release restores forward to the full batch-1/2/3 cleanup with the fix.