templates: Apply saved theme in base template

Read the saved theme from localStorage on page load and set the
data-theme attribute early to avoid a flash of unstyled content.

Signed-off-by: Andrea Cervesato <andrea.cervesato@mailbox.org>
Assisted-by: Gemini 3.8 Flash

Author: acerv

templates/base.html

@@ -2,6 +2,16 @@
 <!DOCTYPE html>
 <html xmlns="http://www.w3.org/1999/xhtml" lang="en">
   <head>
+    <script>
+      (function() {
+        try {
+          var theme = localStorage.getItem('pw_theme');
+          if (theme === 'dark') {
+            document.documentElement.setAttribute('data-theme', 'dark');
+          }
+        } catch (e) {}
+      })();
+    </script>
     <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
     <title>{% block title %}Patchwork{% endblock %} - Patchwork</title>
     <link rel="stylesheet" type="text/css" href="{% static "css/bootstrap.min.css" %}"/>