feat: add setup impact preview UI with delta badges across all views

Adds SetupImpactSelector dropdown and ImpactDeltaBadge inline badge, wired into the thread detail page. Delta badges appear on CandidateListItem, CandidateCard, and ComparisonTable (Weight Impact / Price Impact rows) whenever a setup is selected for comparison.

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

src/client/components/ImpactDeltaBadge.tsx

@@ -0,0 +1,39 @@
+import type { CandidateDelta } from "../hooks/useImpactDeltas";
+
+interface ImpactDeltaBadgeProps {
+	delta: CandidateDelta | undefined;
+	type: "weight" | "price";
+	formatFn: (value: number) => string;
+}
+
+export function ImpactDeltaBadge({
+	delta,
+	type,
+	formatFn,
+}: ImpactDeltaBadgeProps) {
+	if (!delta || delta.mode === "none") return null;
+
+	const value = type === "weight" ? delta.weightDelta : delta.priceDelta;
+
+	if (value === null) {
+		return <span className="text-xs text-gray-400">—</span>;
+	}
+
+	if (value === 0) {
+		return <span className="text-xs text-gray-400">±0</span>;
+	}
+
+	if (value > 0) {
+		return (
+			<span className="text-xs text-green-600">
+				+{formatFn(value)}
+				{delta.mode === "add" && (
+					<span className="ml-0.5 text-green-500">(add)</span>
+				)}
+			</span>
+		);
+	}
+
+	// value < 0
+	return <span className="text-xs text-red-500">{formatFn(value)}</span>;
+}