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/partials/printer_edit_modal.html

@@ -70,7 +70,7 @@
                 {% for item in driver_data %}
                 <option value="{{ item.driver.id }}"
                         {% if p.driver_id == item.driver.id %}selected{% endif %}>
-                  {{ item.driver.original_filename }}{% if item.names %} — {{ item.names | join(', ') }}{% endif %}
+                  {{ item.driver.label }}{% if item.names %} — {{ item.names | join(', ') }}{% endif %}
                 </option>
                 {% endfor %}
               </select>