Add sub-component support via self-referential device hierarchy

templates/device_add.html

@@ -2,10 +2,16 @@
 {% block title %}Add Device — Battery Tracker{% endblock %}
 
 {% block content %}
-<h1>Add Device</h1>
+<h1>{% if prefill_parent %}Add Sub-component to {{ prefill_parent.name }}{% else %}Add Device{% endif %}</h1>
 
 <div class="card">
   <form method="post" action="{{ url_for('device_add') }}">
+    {% if prefill_parent %}
+    <input type="hidden" name="parent_id" value="{{ prefill_parent.id }}">
+    <p class="text-muted" style="margin-bottom:0.75rem;">
+      Adding as a sub-component of <a href="{{ url_for('device_detail', device_id=prefill_parent.id) }}">{{ prefill_parent.name }}</a>.
+    </p>
+    {% endif %}
     <div class="form-group">
       <label for="name">Device Name <span class="text-danger">*</span></label>
       <input type="text" id="name" name="name" value="{{ form_name|default('') }}"