feat(07-01): create useWeightUnit convenience hook

- Wraps useSetting("weightUnit") with typed WeightUnit return
- Validates stored value against known units (g, oz, lb, kg)
- Defaults to "g" when no setting exists (backward compatible)
- Fix config.json formatting (tabs per biome config)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/client/hooks/useWeightUnit.ts

@@ -0,0 +1,12 @@
+import type { WeightUnit } from "../lib/formatters";
+import { useSetting } from "./useSettings";
+
+const VALID_UNITS: WeightUnit[] = ["g", "oz", "lb", "kg"];
+
+export function useWeightUnit(): WeightUnit {
+	const { data } = useSetting("weightUnit");
+	if (data && VALID_UNITS.includes(data as WeightUnit)) {
+		return data as WeightUnit;
+	}
+	return "g";
+}