feat(insights): scaffold 6 metric sub-pages with shared layout shell

Carves the v1.4.24 monolithic /insights page into routed sub-pages
under /insights/{blutdruck,gewicht,puls,stimmung,medikamente,bmi}.
Each sub-page renders its metric chart with the per-chart cog
(chartKey from CHART_OVERLAY_KEYS) and the existing AI status card;
empty states surface a CTA where the chart can't compute (BMI without
height, mood with zero entries, medication adherence without a tracked
medication).

The sticky tab strip moves into src/app/insights/layout.tsx via the
new <InsightsLayoutShell>, which also owns the advisor query so the
regenerate affordance stays reachable from every Insights surface.
The tab strip itself is now route-aware (<Link> + usePathname()) and
exposes an "Overview" pill for the mother page. The CoachDrawer stays
in the mother-page body only — per Marc's directive, navigating to a
sub-page must unmount the drawer.

Shared infrastructure:
  - src/lib/insights/sub-page-metric.ts: slug → MeasurementType[] map
    + SUB_PAGE_SLUGS canonical order.
  - src/hooks/use-insights-layout-prefs.ts: reader for the dashboard-
    layout payload so every sub-page picks up the comparisonBaseline
    without re-fetching.
  - src/components/insights/sub-page-shell.tsx: title + optional badge
    + focusable h1 (a11y — screen-reader focus restoration on tab
    navigation).
  - src/components/insights/insights-layout-shell.tsx: the layout's
    client shell that mounts the strip + threads the regenerate handler.

The 7th sub-page (/insights/schlaf) ships in the follow-up W4c commit
together with its sleep-stage chart components.

src/lib/insights/sub-page-metric.ts

@@ -0,0 +1,61 @@
+/**
+ * v1.4.25 W4 — slug → metric mapping for the routed insights sub-pages.
+ *
+ * The mother page (`/insights`) hosts the overview (hero + briefing +
+ * trends + advisor). Each named slug below is a sub-route that focuses
+ * one metric. Charts on the sub-pages still use the dashboard chart-cog
+ * surface — see `chartKey` strings in `src/lib/dashboard-layout.ts`
+ * (`CHART_OVERLAY_KEYS`). The slugs themselves are German because the
+ * product surface is bilingual and German is the primary author voice.
+ *
+ * `medikamente` carries no `MeasurementType[]` because medication
+ * compliance is event-driven (`MedicationIntakeEvent` rows) rather than
+ * time-series `Measurement` rows; the page fetches that data via the
+ * existing `/api/insights/comprehensive` consumer.
+ *
+ * `bmi` lists both `WEIGHT` + `BODY_HEIGHT` because BMI is derived
+ * client-side — `WEIGHT / (heightCm/100)^2` — and the chart sets
+ * `valueMode="bmi"` on `<HealthChart>`. `BODY_HEIGHT` is informational
+ * only (we don't fetch a height series; the user's height lives on the
+ * profile).
+ */
+export type SubPageSlug =
+  | "blutdruck"
+  | "gewicht"
+  | "puls"
+  | "stimmung"
+  | "medikamente"
+  | "bmi"
+  | "schlaf";
+
+export const SUB_PAGE_SLUGS = [
+  "blutdruck",
+  "gewicht",
+  "puls",
+  "stimmung",
+  "medikamente",
+  "bmi",
+  "schlaf",
+] as const satisfies readonly SubPageSlug[];
+
+export const SUB_PAGE_METRIC: Record<SubPageSlug, string[]> = {
+  blutdruck: ["BLOOD_PRESSURE_SYS", "BLOOD_PRESSURE_DIA", "PULSE"],
+  gewicht: ["WEIGHT"],
+  puls: ["PULSE"],
+  stimmung: ["MOOD"],
+  // medication adherence is event-driven; no measurement series.
+  medikamente: [],
+  // BMI is derived from WEIGHT + profile height (no separate series).
+  bmi: ["WEIGHT"],
+  schlaf: ["SLEEP_DURATION"],
+};
+
+/**
+ * Mother-page route. Kept here as a named constant so call sites
+ * never have to hard-code the string.
+ */
+export const INSIGHTS_OVERVIEW_PATH = "/insights" as const;
+
+export function isSubPageSlug(value: string): value is SubPageSlug {
+  return (SUB_PAGE_SLUGS as readonly string[]).includes(value);
+}