fix(dashboard): show the blood pressure axis in mmHg, not Hg

The dashboard's blood pressure chart passed `yAxisUnit="Hg"` beside
`unit="mmHg"`, a shortening carried since the first release, so every
tick read "154 Hg". The override is gone; "mmHg" is four characters and
fits the axis gutter like "bpm" and "mg/dL" do.

A unit with a space of its own ("/ 5") is now joined with no-break
spaces as well, so Recharts cannot wrap it either. A guard fails on any
`yAxisUnit` that is a cut-down form of the `unit` beside it.

src/app/page-client.tsx

@@ -1780,7 +1780,6 @@ export default function DashboardPageClient({
                 title={t("dashboard.bloodPressure")}
                 colors={["var(--chart-3)", "var(--chart-4)"]}
                 unit="mmHg"
-                yAxisUnit="Hg"
                 targetZones={bpTargetZones}
                 compareBaseline={compareBaseline}
                 userTimezone={user?.timezone}