fix(dashboard): top-tile selector actually filters the tile-strip

Marc reported the toggles in Settings → Dashboard didn't filter
the dashboard tile-strip on reload — the v1.4.15 A4 claim was
correct in spirit but broken in the API contract.

Root cause: `widgetIdEnum` in
`src/app/api/dashboard/widgets/route.ts` was missing
`achievements` even though the default layout has it. Every PUT
that included the achievements widget therefore failed schema
validation with 422 — the toast surfaced "Layout konnte nicht
gespeichert werden" but the user never noticed because the toggle
visually flipped (local draft state) before the network call. On
reload the saved layout was the previous one, so the filter looked
like a no-op.

Fix:
- Extract `DASHBOARD_WIDGET_IDS` as the single source of truth in
  `src/lib/dashboard-layout.ts`. The Zod enum in the API route is
  built from it so the two lists cannot drift again.
- Hide the tile-strip wrapper entirely when the user toggled off
  every tile (Marc's constraint: "immer die gesamte Spalte breit
  und immer der gleichen Höhe ausfüllen" — an empty grid wrapper
  was leaving a thin gap on the page).

Tests: contract test pinning that every widget id in
DEFAULT_DASHBOARD_LAYOUT round-trips through serialize → resolve,
plus an explicit assertion that `achievements` is in the default
layout.

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

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

@@ -13,27 +13,24 @@ import {
   resolveDashboardLayout,
   serializeDashboardLayout,
   DEFAULT_DASHBOARD_LAYOUT,
+  DASHBOARD_WIDGET_IDS,
   type DashboardLayout,
 } from "@/lib/dashboard-layout";
 import { Prisma } from "@/generated/prisma/client";
 import { z } from "zod/v4";
 import type { NextRequest } from "next/server";
 
-const widgetIdEnum = z.enum([
-  "weight",
-  "bp",
-  "pulse",
-  "bodyFat",
-  "mood",
-  "medications",
-  "sleep",
-  "steps",
-  "glucose",
-  "totalBodyWater",
-  "boneMass",
-  "bpInTarget",
-  "oxygenSaturation",
-]);
+// Single source of truth — every widget id rendered by the Settings →
+// Dashboard UI (`src/components/settings/dashboard-layout-section.tsx`
+// iterates the full layout from `DEFAULT_DASHBOARD_LAYOUT`). Missing
+// one here makes the PUT 422 silently — the toast surfaces "Layout
+// konnte nicht gespeichert werden" — and Marc's tile-toggle looks
+// like it does nothing because the save round-trip never completes.
+// v1.4.16 A5 root-cause: `achievements` was absent from this enum so
+// every save attempted with the achievements widget present (i.e.
+// every save against the default layout) was rejected. We now derive
+// the enum from `DASHBOARD_WIDGET_IDS` so the two lists cannot drift.
+const widgetIdEnum = z.enum(DASHBOARD_WIDGET_IDS);
 
 const layoutSchema = z.object({
   version: z.literal(1),