- Remove the non-functional PeriodSelector from /reports/cartes — the Cartes
report is by design a "month X vs X-1 vs X-12" snapshot, so the
reference-month picker is the only control needed.
- Simplify useCartes to drop its useReportsPeriod dependency; the hook now
only exposes the reference year/month and its setter.
- Add a (?) help bubble (lucide HelpCircle) next to the savings-rate KPI
title, wired up via a new `tooltip?: string` prop on KpiCard.
- Propagate `number | null` through CartesKpi.current and buildKpi so
savings rate is now null (rendered as "—") when reference-month income
is 0 instead of a misleading "0 %". Use refExpenses directly for
seasonality.referenceAmount since it is always numeric.
- Update the cartes snapshot tests to expect null for the zero-income case.
- Add FR/EN strings reports.cartes.savingsRateTooltip + CHANGELOG entries
in both locales.
