theme: pin the resolved theme before paint; pill label waits for mount

app/layout.tsx

@@ -29,11 +29,14 @@ export const metadata = {
 
 /**
  * Resolves the theme before first paint: stored choice wins, else system.
- * Keeps the HTML attribute semantics of styles/tokens.css — "dark" set
- * explicitly, "light" set explicitly (to opt out of the media query),
- * nothing when the reader has no JS (the media query covers them).
+ * Always pins the resolved value — including "light", which must be set
+ * explicitly to opt out of the prefers-color-scheme fallback in
+ * styles/tokens.css (leaving it unpinned paints dark until hydration on
+ * dark-system machines whose reader chose light). Nothing is set when the
+ * reader has no JS… except there is no "when": this script only runs with
+ * JS. The no-JS story is the media query alone, untouched.
  */
-const themeBootstrap = `(function(){try{var t=localStorage.getItem('pln:theme');if(t!=='light'&&(t==='dark'||window.matchMedia('(prefers-color-scheme: dark)').matches)){document.documentElement.dataset.theme='dark'}}catch(e){}})()`;
+const themeBootstrap = `(function(){try{var t=localStorage.getItem('pln:theme')||'auto';document.documentElement.dataset.theme=t==='auto'?(window.matchMedia('(prefers-color-scheme: dark)').matches?'dark':'light'):t}catch(e){}})()`;
 
 export default function RootLayout({ children }: { children: React.ReactNode }) {
   return (

components/ThemeSwitcher.jsx

@@ -20,14 +20,17 @@ const LABELS = {
  * this component only manages the cycle and the persistence.
  */
 export default function ThemeSwitcher() {
-  const [mode, setMode] = useState('auto');
+  // Null until mounted: SSR would paint the label "auto" even for a reader
+  // whose stored mode is light/dark — the glimpse this pill used to flash.
+  // An empty pill for a frame beats a wrong label.
+  const [mode, setMode] = useState(null);
 
   useEffect(() => {
     try {
       const stored = window.localStorage.getItem(KEY);
-      if (MODES.includes(stored)) setMode(stored);
+      setMode(MODES.includes(stored) ? stored : 'auto');
     } catch {
-      // storage refused — the pill still cycles, unaided
+      setMode('auto'); // storage refused — the pill still cycles, unaided
     }
     // Follow the system while in auto.
     const media = window.matchMedia('(prefers-color-scheme: dark)');
@@ -68,13 +71,17 @@ export default function ThemeSwitcher() {
       type="button"
       onClick={cycle}
       className={styles.pill}
-      aria-label={LABELS[mode]}
-      title={LABELS[mode]}
+      aria-label={LABELS[mode] ?? 'Thème'}
+      title={LABELS[mode] ?? 'Thème'}
     >
+      {mode && (
+        <>
           <span aria-hidden="true" className={styles.icon}>
             {ICONS[mode]}
           </span>
           <span className={styles.mode}>{mode}</span>
+        </>
+      )}
     </button>
   );
 }