feat: BigNumber fixed-point decimal safety wrapper for multi-tariff scaling.

[real-venus]

BigNumber Fixed-Point Arithmetic Safety Wrapper for Multi-Tariff Decimal Scaling (#53)

Closes #53

What's included

File	Responsibility
src/utils/decimal.ts	Branded Decimal<P> over a cloned BigNumber (28 sig digits, ROUND_HALF_UP — no global-config mutation). Factory decimal(value, precision) + add/sub/mul/div/min/max/sum/average/neg/abs and eq/gt/gte/lt/lte. Overflow past MAX_SAFE (10²¹) throws SafetyRangeError; unsupported precision / bad input throws PrecisionError. toRedux()/fromRedux() give lossless string serialization; toDecimalNumber() warns on precision loss.
src/types/meter.ts	MeterReading / ResourceConsumption / TariffRate keyed to Decimal, with canonical precision per resource (water 0, cost 2, electricity 3, gas 4, submeter 6, nano 9).
src/utils/aggregation.ts	sumReadings, averageReadings, min/maxReading, cumulativeTotals, aggregateByWindow, and applyTariff — Decimal end-to-end, never cast back to number mid-pipeline.
src/store/slices/resourceSlice.ts	Store keeps readings as string-encoded SerializedDecimal (JSON/DevTools-safe) and rehydrates Decimals only inside selectors.
src/components/dashboard/ResourceChart.tsx	SVG time-series that converts each value via toDecimalNumber once at render, while the headline total stays exact (toFixed).
src/hooks/useResourceData.ts	Converts the API's string-encoded decimals into Decimal at the resource's precision on ingestion (never parsed as number).
tests/unit/{decimal,aggregation,resourceSlice}.test.ts	Mixed-precision add upcast, mul overflow, serialization round-trip, 1000-iteration zero-drift, tariff rounding, and store round-trip.