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/frontend/components/Sidebar.tsx

@@ -5,6 +5,7 @@ import { usePathname } from "next/navigation";
 import { useEffect, useState } from "react";
 import { listProjects } from "../utils/api";
 import type { ProjectDto } from "../types";
+import { ThemeToggle } from "./ThemeToggle";
 
 const PROJECT_SECTIONS = [
   { id: "verlauf", label: "Verlauf", icon: "↺" },
@@ -116,6 +117,9 @@ export function Sidebar() {
           </>
         )}
       </nav>
+      <div className="sidebar-footer">
+        <ThemeToggle />
+      </div>
     </aside>
   );
 }