Add a dark theme, default, with a switch in the top bar

Colours move into theme tokens (light values in :root, dark in
[data-theme=dark]); the closing contact band inverts on each theme. The
choice is stored in localStorage and applied by a head script before the
first paint. Voxel covers and image shadows follow the theme.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XtcZtnG4GQA1d6mW5PNrS9

components/ThemeToggle.tsx

@@ -0,0 +1,47 @@
+'use client'
+
+import { useEffect, useState } from 'react'
+import styles from './topbar.module.css'
+
+type Theme = 'dark' | 'light'
+
+/** Switches between the dark (default) and light theme; the choice is remembered in this browser. */
+export default function ThemeToggle() {
+  const [theme, setTheme] = useState<Theme>('dark')
+
+  // The page starts dark; a small script in <head> applies a stored "light" choice before the first paint.
+  useEffect(() => {
+    setTheme(document.documentElement.dataset.theme === 'light' ? 'light' : 'dark')
+  }, [])
+
+  const toggle = () => {
+    const next: Theme = theme === 'dark' ? 'light' : 'dark'
+    document.documentElement.dataset.theme = next
+    try {
+      localStorage.setItem('theme', next)
+    } catch {
+      // Storage can be blocked; the theme still switches for this visit.
+    }
+    setTheme(next)
+  }
+
+  return (
+    <button
+      type="button"
+      className={styles.toggle}
+      onClick={toggle}
+      aria-label={theme === 'dark' ? 'Switch to the light theme' : 'Switch to the dark theme'}
+    >
+      {theme === 'dark' ? (
+        <svg viewBox="0 0 24 24" aria-hidden="true">
+          <circle cx="12" cy="12" r="4" />
+          <path d="M12 2.5v2.2M12 19.3v2.2M4.2 4.2l1.6 1.6M18.2 18.2l1.6 1.6M2.5 12h2.2M19.3 12h2.2M4.2 19.8l1.6-1.6M18.2 5.8l1.6-1.6" />
+        </svg>
+      ) : (
+        <svg viewBox="0 0 24 24" aria-hidden="true">
+          <path d="M20.5 14.2A8.5 8.5 0 0 1 9.8 3.5a8.5 8.5 0 1 0 10.7 10.7z" />
+        </svg>
+      )}
+    </button>
+  )
+}