feat: add weather unit preferences

lib/forecast-utils.ts

@@ -3,11 +3,13 @@ import { translate } from "@/lib/i18n";
 import {
   DEFAULT_TEMPERATURE_UNIT,
   DEFAULT_WIND_SPEED_UNIT,
+  DEFAULT_PRECIPITATION_UNIT,
+  formatPrecipitation,
   formatTemperature,
   formatWindSpeed,
 } from "@/lib/weather-utils";
 import type { DailyForecast, HourlyForecast } from "@/types/forecast";
-import type { TemperatureUnit, WindSpeedUnit } from "@/types/units";
+import type { PrecipitationUnit, TemperatureUnit, WindSpeedUnit } from "@/types/units";
 
 export function getForecastConditionKey(code: number | null): TranslationKey {
   if (code === 0) return "forecast.condition.clear";
@@ -47,9 +49,13 @@ export function formatForecastTemperature(
   return formatTemperature(value, language, unit);
 }
 
-export function formatForecastRainfall(value: number | null, language: Language) {
+export function formatForecastRainfall(
+  value: number | null,
+  language: Language,
+  unit: PrecipitationUnit = DEFAULT_PRECIPITATION_UNIT,
+) {
   if (value === null) return "—";
-  return `${new Intl.NumberFormat(language === "pl" ? "pl-PL" : "en-GB", { maximumFractionDigits: 1 }).format(value)} mm`;
+  return formatPrecipitation(value, language, unit);
 }
 
 export function formatForecastWind(