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/hooks/useThreads.ts

@@ -40,6 +40,7 @@ interface ThreadWithCandidates {
 	name: string;
 	status: "active" | "resolved";
 	resolvedCandidateId: number | null;
+	categoryId: number;
 	createdAt: string;
 	updatedAt: string;
 	candidates: CandidateWithCategory[];