feat(comparison): comparison toggle persists baseline preference

Add a "Compare to" pulldown to the Settings → Dashboard section that
lets the user pick None / Last month / Last year. The preference rides
on the existing User.dashboardWidgetsJson blob — research §7 Q3 settled
on the JSON-pivot path so v1.4.16 ships migration-free.

The resolver defaults to "none" for legacy layouts and clamps unknown
values back to "none", so a stale client cannot poison the dashboard
with a baseline the renderer doesn't know how to draw. The PUT
endpoint accepts the new optional comparisonBaseline field; pre-B8
clients keep PUTing without it and see no behavioural change.

EN+DE i18n keys land for the toggle label, the three options, the
"unavailable" fallback, and the chart caption — Vormonat / Vorjahr per
Marc's brief.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/api/dashboard/widgets/route.ts

@@ -14,6 +14,7 @@ import {
   serializeDashboardLayout,
   DEFAULT_DASHBOARD_LAYOUT,
   DASHBOARD_WIDGET_IDS,
+  COMPARISON_BASELINES,
   type DashboardLayout,
 } from "@/lib/dashboard-layout";
 import { Prisma } from "@/generated/prisma/client";
@@ -48,6 +49,10 @@ const layoutSchema = z.object({
     )
     .min(1)
     .max(20),
+  // v1.4.16 phase B8 — comparison baseline (Vormonat / Vorjahr) rides
+  // on the layout blob per research §7 Q3 (no Prisma migration). Optional
+  // so v1.4.15 clients that don't know the field can still PUT.
+  comparisonBaseline: z.enum(COMPARISON_BASELINES).optional(),
 });
 
 export const GET = apiHandler(async () => {