feat: add user theme preference

- Add theme field to User model (common)
- Create migration for theme field
- Add theme selection to Account Settings (html-router)
- Implement server-side theme rendering in base template
- Update JS for system/preference theme handling
- Remove header theme toggle for authenticated users

html-router/templates/navigation_bar.html

@@ -8,5 +8,7 @@
   <label for="my-drawer" aria-label="open sidebar" class="hover:cursor-pointer lg:hidden">
     {% include "icons/hamburger_icon.html" %}
   </label>
+  {% if not is_authenticated %}
   {% include "theme_toggle.html" %}
+  {% endif %}
 {% endblock %}