fix(settings): dashboard section save actually persists

The Settings → Dashboard sub-section's Save button silently 422'd once
the user had touched any per-chart overlay popover. Root cause was a
Zod v4 semantics change: `z.record(z.enum(KEYS), ...)` now requires
every enum key to be present in the input, where Zod v3 treated the
same shape as a partial record. Real-world Save payloads only carry
the chart keys the user actually toggled, so every Settings → Dashboard
Save attempt with one or two `chartOverlayPrefs` entries failed with
`expected: object, path: ["chartOverlayPrefs", "<missing-key>"]` and
the toast surfaced "Layout konnte nicht gespeichert werden".

Fix uses `z.partialRecord(z.enum(KEYS), ...)` which preserves the
original v1.4.18 intent — per-chart opt-in overlay prefs, missing keys
filled by the resolver. The inner object now also accepts the
`comparisonBaseline` field so a Settings save preserves the per-chart
comparison toggle the user set via the chart-card popover (otherwise
Zod's default strip behaviour wiped it on every save).

New integration suite pins:
  - partial chartOverlayPrefs round-trips
  - per-chart comparisonBaseline survives a Settings save
  - empty chartOverlayPrefs still persists (fresh accounts)
  - widget toggle changes land in the DB

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

@@ -58,13 +58,36 @@ const layoutSchema = z.object({
   // v1.4.18 — per-chart overlay prefs (3 toggles per chart card).
   // Optional so older clients that don't know the field can still PUT;
   // the resolver coerces malformed values away from the layout blob.
+  //
+  // v1.4.25 W6 — switched from `z.record(enum, …)` to `z.partialRecord(…)`
+  // because Zod v4 changed the semantics of `z.record(enum, …)`: it now
+  // requires every enum key to be present (a breaking change from
+  // Zod v3 which behaved like a partial record). With the strict variant
+  // any PUT that carried `chartOverlayPrefs` with fewer than ALL nine
+  // chart keys (i.e. every real-world Save click once a user had touched
+  // a per-chart overlay popover) 422'd with
+  // `expected: object, path: ["chartOverlayPrefs", "<missing-key>"]` and
+  // surfaced as the toast `Layout konnte nicht gespeichert werden`.
+  // Partial-record matches the original intent — overlay prefs are
+  // per-chart opt-in, the resolver fills in defaults for missing keys.
+  //
+  // The inner object also documents `comparisonBaseline` so the
+  // per-chart `<ChartOverlayControls>` popover (which calls
+  // `/api/dashboard/chart-overlay-prefs`) and a full-layout PUT from
+  // Settings → Dashboard can both round-trip the field. Without it Zod
+  // would silently strip the per-chart `comparisonBaseline` on every
+  // Save click in Settings, wiping any per-chart toggle the user had
+  // set via the chart-card popover.
   chartOverlayPrefs: z
-    .record(
+    .partialRecord(
       z.enum(CHART_OVERLAY_KEYS),
       z.object({
         showTrendIndicator: z.boolean(),
         showTrendArrow: z.boolean(),
         showTargetRange: z.boolean(),
+        comparisonBaseline: z
+          .enum(["none", "lastMonth", "lastYear"])
+          .optional(),
       }),
     )
     .optional(),