fix: theme toggle now properly handles auto mode

Remove data-theme attribute when mode="auto" instead of setting it to
"auto". Pico's OS-dark selector is :root:not([data-theme]), so any
value (even "auto") prevents dark-mode Pico styles. Add Pico form
element background vars and color-scheme: light to match active
theme.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

imptune/templates/printers_new.html

@@ -65,7 +65,7 @@
               <option value="" x-text="$store.i18n.t('no_driver_option')">No driver</option>
               {% for item in driver_data %}
               <option value="{{ item.driver.id }}">
-                {{ item.driver.original_filename }}{% if item.names %} — {{ item.names | join(', ') }}{% endif %}
+                {{ item.driver.label }}{% if item.names %} — {{ item.names | join(', ') }}{% endif %}
               </option>
               {% endfor %}
             </select>