Scoped useReducer hook backing BalancePage. Tracks:
- period (3M / 6M / 1A / 3A / all) — defaults to 1A
- chartMode (line / stacked) — defaults to line
- evolutionTotals + evolutionByCategory + accountsLatest +
accountsPeriodAnchor (parallel-fetched on mount and on every
period change via Promise.all)
- isLoading + error
Exposes computeBalanceDateRange(period, today) as a pure helper
so the date math is unit-testable without mocking time. Anchors
on `today` rather than the latest snapshot — keeps the chart's
right edge stable as the user enters new snapshots.
Refs: #141
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
