fix dark/light theme toggle — was a dead button with no JS handler

Root cause: the [data-theme-toggle] button existed in ASW's baseof.html
but had no JavaScript attached to it — clicking it did nothing.

Fix:
- static/js/theme-toggle.js — click handler, localStorage persistence,
  prefers-color-scheme fallback, dynamic button icon + aria-label
- static/css/garden.css — full html[data-theme="light"] variable block
  (garden + ASW tokens) so light mode actually looks different from dark
- layouts/partials/head.html — include theme-toggle.js with defer

Light palette: near-white indigo bg (96%), dark indigo text (20%),
violet accents. ASW's @media (prefers-color-scheme) doesn't match
programmatic data-theme toggle, so garden.css provides duplicating
overrides on html[data-theme="light"].

static/js/theme-toggle.js

@@ -0,0 +1,71 @@
+/**
+ * theme-toggle.js — dark/light toggle for Vigo's garden.
+ *
+ * Handles the [data-theme-toggle] button in the nav (from ASW's baseof.html).
+ * Persists preference to localStorage. Falls back to prefers-color-scheme,
+ * then to dark (the garden default).
+ */
+
+(function () {
+  'use strict';
+
+  var STORAGE_KEY = 'garden-theme';
+  var HTML = document.documentElement;
+
+  // ── Icons ──────────────────────────────────────────────────────────
+  var ICONS = {
+    dark: '\u{1F31B}',  // crescent moon — current theme is light, click to go dark
+    light: '\u{2600}\u{FE0F}',  // sun — current theme is dark, click to go light
+  };
+
+  // ── Labels (accessibility) ─────────────────────────────────────────
+  var LABELS = {
+    dark: 'Switch to dark theme',
+    light: 'Switch to light theme',
+  };
+
+  // ── Resolve initial theme ──────────────────────────────────────────
+  function getInitialTheme() {
+    var saved = localStorage.getItem(STORAGE_KEY);
+    if (saved === 'dark' || saved === 'light') return saved;
+    if (window.matchMedia && window.matchMedia('(prefers-color-scheme: light)').matches) {
+      return 'light';
+    }
+    return 'dark';  // garden default
+  }
+
+  // ── Apply theme ────────────────────────────────────────────────────
+  function applyTheme(theme, button) {
+    HTML.setAttribute('data-theme', theme);
+    var other = theme === 'dark' ? 'light' : 'dark';
+    if (button) {
+      button.textContent = ICONS[other];
+      button.setAttribute('aria-label', LABELS[other]);
+    }
+  }
+
+  // ── Boot ───────────────────────────────────────────────────────────
+  function init() {
+    var button = document.querySelector('[data-theme-toggle]');
+    if (!button) return;
+
+    // Apply saved/system theme on load
+    var theme = getInitialTheme();
+    applyTheme(theme, button);
+    localStorage.setItem(STORAGE_KEY, theme);
+
+    // Click handler
+    button.addEventListener('click', function () {
+      var current = HTML.getAttribute('data-theme') || 'dark';
+      var next = current === 'dark' ? 'light' : 'dark';
+      applyTheme(next, button);
+      localStorage.setItem(STORAGE_KEY, next);
+    });
+  }
+
+  if (document.readyState === 'loading') {
+    document.addEventListener('DOMContentLoaded', init);
+  } else {
+    init();
+  }
+})();