Add dark mode toggle in the sidebar

Adds a persistent dark/light theme toggle (localStorage +
data-bs-theme on <html>) so it applies globally, including the
circuit editor route which has no visible sidebar. An inline
beforeInteractive script sets the theme before hydration to avoid a
flash of the wrong theme.

Bootstrap components pick up data-bs-theme automatically; the custom
circuit-grid CSS previously used ~150 hardcoded hex colors, all
converted to semantic tokens in globals.css with light/dark values.

Verified with a headless-browser pass across the project list, project
detail page, circuit editor, project-device drawer and a settings
modal in both themes; no console errors.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/layout.tsx

@@ -1,4 +1,5 @@
 import type { Metadata } from "next";
+import Script from "next/script";
 import "bootstrap/dist/css/bootstrap.min.css";
 import "./globals.css";
 import { AppShell } from "../frontend/components/AppShell";
@@ -8,10 +9,33 @@ export const metadata: Metadata = {
   description: "Leistungsbilanz für elektrische Verbraucher und Stromkreislisten",
 };
 
+// Keep this key literal in sync with THEME_STORAGE_KEY in ThemeToggle.tsx.
+// It must stay inline (not imported) so it runs before hydration and never
+// flashes the wrong theme on load.
+const THEME_INIT_SCRIPT = `
+(function () {
+  try {
+    var stored = localStorage.getItem("leistungsbilanz:theme");
+    var theme =
+      stored === "dark" || stored === "light"
+        ? stored
+        : window.matchMedia("(prefers-color-scheme: dark)").matches
+          ? "dark"
+          : "light";
+    document.documentElement.setAttribute("data-bs-theme", theme);
+  } catch (error) {
+    // Storage/matchMedia unavailable: fall back to the default light theme.
+  }
+})();
+`;
+
 export default function RootLayout({ children }: Readonly<{ children: React.ReactNode }>) {
   return (
-    <html lang="de">
+    <html lang="de" suppressHydrationWarning>
       <body>
+        <Script id="theme-init" strategy="beforeInteractive">
+          {THEME_INIT_SCRIPT}
+        </Script>
         <AppShell>{children}</AppShell>
       </body>
     </html>