refactor(ui): route the sleep-composition header through the tile header

The stage-composition chart card was the last insights tile with a
hand-rolled title row. It now uses the canonical header primitive; the
subtitle and the window toggles keep the shape they had, since the toggles
have to drop to their own row on a phone.

Also sweeps the last off-scale spacing: the two dense tiles inside a
recommendation move onto the sanctioned inner-tile step, and the not-found
button onto the scale. The banned twenty-pixel step is now absent from the
tree. The chart body is untouched.

src/app/not-found.tsx

@@ -49,7 +49,7 @@ export default async function NotFound() {
       </div>
       <Link
         href="/"
-        className="bg-primary text-primary-foreground hover:bg-primary/90 inline-flex min-h-11 items-center justify-center rounded-md px-5 text-sm font-medium transition-colors"
+        className="bg-primary text-primary-foreground hover:bg-primary/90 inline-flex min-h-11 items-center justify-center rounded-md px-4 text-sm font-medium transition-colors"
       >
         {t("notFound.backToDashboard")}
       </Link>

src/components/insights/__tests__/sleep-stage-stacked-bar.test.tsx

@@ -51,6 +51,9 @@ describe("<SleepStageStackedBar>", () => {
     const html = render(<SleepStageStackedBar breakdown={breakdown} />);
     expect(html).toContain("Stage composition");
     expect(html).toContain("Last 23 nights");
+    // The title routes through the canonical tile header rather than a
+    // hand-rolled title row (UI-STANDARDS §1 / §8).
+    expect(html).toContain('data-slot="tile-header"');
   });
 
   it("exposes an accessible label that announces the nights covered", () => {

src/components/insights/recommendation-card.tsx

@@ -240,7 +240,7 @@ function RationaleCard({
   return (
     <div
       data-slot="rec-rationale-card"
-      className="bg-muted/30 mt-3 space-y-3 rounded-md px-3 py-2.5"
+      className="bg-muted/30 mt-3 space-y-3 rounded-md p-3"
     >
       <div className="space-y-1.5">
         <RationaleRow
@@ -358,7 +358,7 @@ export function RecommendationCard({
     <div
       data-slot="rec-card"
       data-index={index}
-      className="border-border/40 bg-card/20 rounded-lg border px-3 py-2.5"
+      className="border-border/40 bg-card/20 rounded-lg border p-3"
     >
       <div className="flex items-start justify-between gap-2">
         <div className="flex min-w-0 flex-1 items-start gap-2">

src/components/insights/sleep-stage-stacked-bar.tsx

@@ -14,7 +14,8 @@ import {
 } from "recharts";
 
 import { Button } from "@/components/ui/button";
-import { Card, CardContent, CardHeader, CardTitle } from "@/components/ui/card";
+import { Card, CardContent, CardHeader } from "@/components/ui/card";
+import { TileHeader } from "@/components/insights/tile-header";
 import { useTranslations } from "@/lib/i18n/context";
 import { formatDurationMinutes } from "@/lib/i18n/duration";
 import { resolveIntlLocale } from "@/lib/format-locale";
@@ -205,10 +206,14 @@ export function SleepStageStackedBar({ breakdown }: SleepStageStackedBarProps) {
     <Card data-slot="sleep-stage-stacked-bar">
       <CardHeader>
         <div className="flex flex-col gap-2 sm:flex-row sm:items-center sm:justify-between">
-          <div className="flex flex-col gap-0.5">
-            <CardTitle className="text-base font-semibold">
-              {t("insights.sleep.compositionTitle")}
-            </CardTitle>
+          <div className="flex min-w-0 flex-col gap-0.5">
+            {/* The canonical tile header — the title used to be a bare
+                `CardTitle` in a hand-rolled row. The window toggles stay a
+                sibling of the title+subtitle pair rather than riding the
+                header's `right` slot: the subtitle belongs under the title,
+                and the toggles need to drop to their own row below `sm`
+                (the sanctioned inline-action-row shape). */}
+            <TileHeader title={t("insights.sleep.compositionTitle")} />
             <span className="text-muted-foreground text-xs">
               {t("insights.sleep.compositionSubtitle", {
                 nights: breakdown.nights,