fix(dashboard): stop the chart-row render loop on tab resume

A user reported the dashboard crashing on returning to a backgrounded
tab. The on-focus snapshot refetch re-rendered the page, and each chart
received a fresh inline `() => markChartReady(id)` handler every render;
the chart's data-ready notify effect, keyed on that ever-changing prop,
re-fired on every commit. That per-commit passive effect kept the chart
row's anchored popovers re-committing until the browser tripped its
render-depth guard and the page fell back to its error card.

Latch the notify behind a ref so it fires once, when the chart's initial
query settles, and depend on the load flag alone. The handler is read
through a ref so a late-arriving callback still fires without re-arming
the effect. Extract the pure fire-once decision into a small helper with
direct unit coverage (no DOM runner, per project convention).

package.json

@@ -1,6 +1,6 @@
 {
   "name": "healthlog",
-  "version": "1.20.0",
+  "version": "1.20.1",
   "description": "Self-hosted personal-health-tracking PWA with Withings integration, AI insights, and doctor-report PDF export.",
   "license": "PolyForm-Noncommercial-1.0.0",
   "homepage": "https://healthlog.dev",