feat: add Jinja2 admin UI templates

app/templates/config_form.html

@@ -1 +1,20 @@
-{% extends "base.html" %}{% block content %}form{% endblock %}
+{% extends "base.html" %}
+{% block content %}
+<h1>{{ "Edit Config" if config else "New Config" }}</h1>
+<div class="card">
+  <form method="post">
+    {% if not config %}
+    <div style="margin-bottom:14px;">
+      <label for="name">Name</label>
+      <input type="text" id="name" name="name" required placeholder="My Config">
+    </div>
+    {% endif %}
+    <div style="margin-bottom:14px;">
+      <label for="base_yaml">Base YAML</label>
+      <textarea id="base_yaml" name="base_yaml" required>{{ config.base_yaml if config else "" }}</textarea>
+    </div>
+    <button type="submit" class="btn btn-primary">Save</button>
+    <a href="/" class="btn" style="background:#6b7280;color:#fff;">Cancel</a>
+  </form>
+</div>
+{% endblock %}