feat(07-02): add weight unit toggle and wire all formatWeight call sites

- Add segmented g/oz/lb/kg toggle to TotalsBar with settings persistence
- Pass unit parameter to all 8 formatWeight call sites across components and routes
- Import useWeightUnit hook in ItemCard, CandidateCard, CategoryHeader, SetupCard, ItemPicker, Dashboard, SetupDetail

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

src/client/components/CandidateCard.tsx

@@ -1,3 +1,4 @@
+import { useWeightUnit } from "../hooks/useWeightUnit";
 import { formatPrice, formatWeight } from "../lib/formatters";
 import { LucideIcon } from "../lib/iconData";
 import { useUIStore } from "../stores/uiStore";
@@ -27,6 +28,7 @@ export function CandidateCard({
 	threadId,
 	isActive,
 }: CandidateCardProps) {
+	const unit = useWeightUnit();
 	const openCandidateEditPanel = useUIStore((s) => s.openCandidateEditPanel);
 	const openConfirmDeleteCandidate = useUIStore(
 		(s) => s.openConfirmDeleteCandidate,
@@ -88,7 +90,7 @@ export function CandidateCard({
 				<div className="flex flex-wrap gap-1.5 mb-3">
 					{weightGrams != null && (
 						<span className="inline-flex items-center px-2 py-0.5 rounded-full text-xs font-medium bg-blue-50 text-blue-400">
-							{formatWeight(weightGrams)}
+							{formatWeight(weightGrams, unit)}
 						</span>
 					)}
 					{priceCents != null && (