feat: add impact delta computation with TDD tests

Implements computeImpactDeltas pure function with 8 TDD tests covering replace/add/none modes and null weight/price handling. Adds useImpactDeltas hook, categoryId to ThreadWithCandidates, and selectedSetupId state to uiStore.

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

src/client/lib/impactDeltas.ts

@@ -0,0 +1,69 @@
+export interface CandidateInput {
+	id: number;
+	weightGrams: number | null;
+	priceCents: number | null;
+}
+
+export interface SetupItemInput {
+	categoryId: number;
+	weightGrams: number | null;
+	priceCents: number | null;
+	name: string;
+}
+
+export type DeltaMode = "replace" | "add" | "none";
+
+export interface CandidateDelta {
+	candidateId: number;
+	mode: DeltaMode;
+	weightDelta: number | null;
+	priceDelta: number | null;
+	replacedItemName: string | null;
+}
+
+export interface ImpactDeltas {
+	mode: DeltaMode;
+	deltas: Record<number, CandidateDelta>;
+}
+
+export function computeImpactDeltas(
+	candidates: CandidateInput[],
+	setupItems: SetupItemInput[] | undefined,
+	threadCategoryId: number,
+): ImpactDeltas {
+	if (!setupItems) return { mode: "none", deltas: {} };
+
+	const replacedItem =
+		setupItems.find((item) => item.categoryId === threadCategoryId) ?? null;
+	const mode: DeltaMode = replacedItem ? "replace" : "add";
+	const deltas: Record<number, CandidateDelta> = {};
+
+	for (const candidate of candidates) {
+		let weightDelta: number | null = null;
+		let priceDelta: number | null = null;
+
+		if (candidate.weightGrams != null) {
+			weightDelta =
+				replacedItem?.weightGrams != null
+					? candidate.weightGrams - replacedItem.weightGrams
+					: candidate.weightGrams;
+		}
+
+		if (candidate.priceCents != null) {
+			priceDelta =
+				replacedItem?.priceCents != null
+					? candidate.priceCents - replacedItem.priceCents
+					: candidate.priceCents;
+		}
+
+		deltas[candidate.id] = {
+			candidateId: candidate.id,
+			mode,
+			weightDelta,
+			priceDelta,
+			replacedItemName: replacedItem?.name ?? null,
+		};
+	}
+
+	return { mode, deltas };
+}