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"].

layouts/partials/head.html

@@ -14,6 +14,7 @@
 <link rel="stylesheet" href="{{ "css/asw.css" | absURL }}">
 <link rel="stylesheet" href="{{ "css/garden.css" | absURL }}">
 
+<script src="{{ "js/theme-toggle.js" | absURL }}" defer></script>
 <script src="{{ "js/garden-feed.js" | absURL }}" defer></script>
 
 {{- range .AlternativeOutputFormats -}}