Implement M1.5 visual design foundation: tokens, IBM Plex, theme toggle, chart themes

src/app/App.tsx

@@ -1,6 +1,7 @@
 import { LivePreview } from './components/LivePreview';
 import { SnippetLibrary } from './components/SnippetLibrary';
 import { SpecEditor } from './components/SpecEditor';
+import { ThemeToggle } from './components/ThemeToggle';
 import styles from './App.module.css';
 
 /**
@@ -18,6 +19,7 @@ export function App() {
         <span className={styles.title}>Astrolabe</span>
         <span className={styles.version}>v{__APP_VERSION__}</span>
         <span className={styles.spacer} />
+        <ThemeToggle />
       </header>
 
       <main className={styles.panes}>