feat: add Jinja2 admin UI templates

app/templates/sub_form.html

@@ -1 +1,19 @@
-{% extends "base.html" %}{% block content %}sub{% endblock %}
+{% extends "base.html" %}
+{% block content %}
+<h1>Add Subscription to "{{ config.name }}"</h1>
+<div class="card">
+  <form method="post">
+    <div style="margin-bottom:14px;">
+      <label for="name">Provider Name</label>
+      <input type="text" id="name" name="name" required
+             placeholder="Must match proxy-provider name in base_yaml">
+    </div>
+    <div style="margin-bottom:14px;">
+      <label for="url">Subscription URL</label>
+      <input type="url" id="url" name="url" required placeholder="https://...">
+    </div>
+    <button type="submit" class="btn btn-primary">Add Subscription</button>
+    <a href="/configs/{{ config.id }}" class="btn" style="background:#6b7280;color:#fff;">Cancel</a>
+  </form>
+</div>
+{% endblock %}