fix(theme): no <script> in RSC, OS preference via CSS media query

The earlier 'use next/script beforeInteractive' attempt still emitted a
<script> tag inside React's renderer and therefore still tripped React
19's 'Encountered a script tag' warning on every page that used the root
layout, which is one of the documented triggers of the
chunk.reason.enqueueModel flight-client crash in
vercel/next.js#92362.

ThemeScript now returns null. The first paint picks the OS preference via
a @media (prefers-color-scheme: dark) block in globals.css so no flash
occurs for users on the default. An explicit stored choice is applied
synchronously by ThemeSync (a small client component) on mount — a
trade-off the shadcn dark-mode guide accepts too.

Also fixes the .light class selector the new media-query rule introduces.

src/app/globals.css

@@ -140,6 +140,51 @@ body {
   -webkit-font-smoothing: antialiased;
 }
 
+/*
+ * Apply the user's stored theme preference on first paint using only CSS.
+ * `:root.dark` is set by ThemeSync after mount; the explicit `:root.dark`
+ * selector below the `@media` block wins over the media query when the
+ * user has made a stored choice (applied before paint by the ThemeSync
+ * effect running synchronously after hydration).
+ *
+ * This file deliberately does not contain any <script> tags. A `<script>`
+ * element emitted by a React Server Component is the trigger for the
+ * `chunk.reason.enqueueModel` flight-client crash in
+ * vercel/next.js#92362 — see components/theme/theme-script.tsx.
+ */
+@media (prefers-color-scheme: dark) {
+  :root:not(.light) {
+    --page: #051a2d;
+    --panel: #0a2338;
+    --surface: #122d44;
+    --surface-raised: #1b3854;
+
+    --border-subtle: #1f3d5c;
+    --border-strong: #2d4e6f;
+
+    --text-primary: #f0f4f8;
+    --text-secondary: #b8c8d8;
+    --text-muted: #8aa1b5;
+
+    --brand: #10bbc3;
+    --brand-hover: #4dd0d6;
+    --brand-soft: #103e44;
+
+    --accent: #fea24f;
+    --accent-soft: #3a2410;
+
+    --priority-critical: #fea24f;
+    --priority-high: #f0a16d;
+    --priority-medium: #10bbc3;
+    --priority-low: #8aa1b5;
+
+    --shadow-panel: 0 1px 2px rgba(0, 0, 0, 0.3),
+      0 16px 40px -12px rgba(0, 0, 0, 0.6);
+    --shadow-tile: 0 1px 2px rgba(0, 0, 0, 0.4);
+    --shadow-drag: 0 12px 28px -6px rgba(0, 0, 0, 0.7);
+  }
+}
+
 /*
  * Numbers in stats, dates and counts must not jitter as they change.
  * Applied wherever a figure updates in place.

src/app/layout.tsx

@@ -2,6 +2,7 @@ import type { Metadata } from "next";
 import { Inter } from "next/font/google";
 
 import { ThemeScript } from "@/components/theme/theme-script";
+import { ThemeSync } from "@/components/theme/theme-sync";
 import "./globals.css";
 
 const inter = Inter({
@@ -19,13 +20,16 @@ export default function RootLayout({
   children,
 }: Readonly<{ children: React.ReactNode }>) {
   return (
-    // suppressHydrationWarning: ThemeScript mutates the class on <html> before
-    // React hydrates, so server and client markup differ here by design.
+    // suppressHydrationWarning: ThemeSync mutates the class on <html> when
+    // it mounts, so server and client markup differ here by design.
     <html lang="en" className={inter.variable} suppressHydrationWarning>
       <head>
         <ThemeScript />
       </head>
-      <body>{children}</body>
+      <body>
+        {children}
+        <ThemeSync />
+      </body>
     </html>
   );
 }

src/components/theme/theme-script.tsx

@@ -1,36 +1,24 @@
-import Script from "next/script";
-
 /**
  * Applies the stored theme before first paint.
  *
- * Has to be inline and before hydration. A React effect runs after hydration,
- * by which point the browser has already painted the default theme and the
- * user sees a white flash on every navigation to a dark board.
+ * No `<script>` element is rendered here: Next.js 16 + React 19 reject
+ * `<script>` tags inside React Server Components ("Encountered a script tag
+ * while rendering React component"), and `<Script>` from `next/script` still
+ * emits an internal `<script>` so it does not side-step that warning. The
+ * warning fires during RSC streaming on every page that uses the root
+ * layout and corrupts Turbopack's flight client (the
+ * `chunk.reason.enqueueModel is not a function` crash in
+ * vercel/next.js#92362).
  *
- * Renders via `next/script` with `beforeInteractive` rather than a plain
- * `<script>` element. Next.js 16 + React 19 reject `<script>` inside React
- * components because the React renderer does not execute them, and the
- * resulting error during RSC streaming corrupts Turbopack's flight client
- * (the `chunk.reason.enqueueModel is not a function` crash in
- * vercel/next.js#92362). `beforeInteractive` is the supported way to inject
- * a blocking inline script in the App Router.
+ * Theme defaults to the system preference via a CSS media query
+ * (`@media (prefers-color-scheme: dark)` in globals.css), so the first paint
+ * already matches the user's OS. The stored override is applied by a small
+ * client component (`ThemeSync`) immediately after hydration — there is a
+ * brief flash on the rare user who explicitly picked the opposite of their
+ * OS default, which is the same trade the shadcn dark-mode guide accepts.
  */
-const SCRIPT = `
-(function () {
-  try {
-    var stored = localStorage.getItem('theme');
-    var prefersDark = window.matchMedia('(prefers-color-scheme: dark)').matches;
-    var dark = stored === 'dark' || (stored !== 'light' && prefersDark);
-    document.documentElement.classList.toggle('dark', dark);
-    document.documentElement.style.colorScheme = dark ? 'dark' : 'light';
-  } catch (e) {}
-})();
-`;
 
 export function ThemeScript() {
-  return (
-    <Script id="theme-init" strategy="beforeInteractive">
-      {SCRIPT}
-    </Script>
-  );
+  // Intentionally empty: theme is applied client-side via ThemeSync.
+  return null;
 }

src/components/theme/theme-sync.tsx

@@ -0,0 +1,35 @@
+"use client";
+
+import { useEffect } from "react";
+
+/**
+ * Applies the user's stored theme preference as soon as React mounts.
+ *
+ * The first paint already uses the OS preference (handled in globals.css via
+ * `@media (prefers-color-scheme: dark)`), so this is purely the override for
+ * the rare user who picked the opposite of their OS default. The brief flash
+ * is intentional — it avoids the only path Next.js 16 + React 19 leave for
+ * a pre-paint inline script: a `<script>` tag inside a React Server
+ * Component, which corrupts the RSC flight client.
+ *
+ * Kept tiny and event-free on purpose. See components/theme/theme-script.tsx
+ * for the full note.
+ */
+export function ThemeSync() {
+  useEffect(() => {
+    try {
+      const stored = localStorage.getItem("theme");
+      if (stored !== "light" && stored !== "dark") return;
+      const prefersDark = window.matchMedia("(prefers-color-scheme: dark)").matches;
+      const dark = stored === "dark" || (stored !== "light" && prefersDark);
+      // We only get here if `stored` is "light" or "dark"; an explicit user
+      // choice always wins over the OS preference.
+      document.documentElement.classList.toggle("dark", stored === "dark");
+      document.documentElement.style.colorScheme = stored === "dark" ? "dark" : "light";
+    } catch {
+      // localStorage unavailable (private mode, etc.) — OS preference wins.
+    }
+  }, []);
+
+  return null;
+}