feat: restyle admin UI and add per-user sync run history

Adds a self-hosted stylesheet (no CDN dependencies) with a card-based
dashboard and color-coded status badges, and shows the last 10 sync
runs per user on the detail page.

app/web/templates/login.html

@@ -4,13 +4,15 @@
 
 {% block content %}
 <h1>Admin Login</h1>
-{% if error %}
-<p class="error">{{ error }}</p>
-{% endif %}
-<form method="post" action="/login">
-    <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
-    <label for="password">Password</label>
-    <input type="password" id="password" name="password" required autofocus>
-    <button type="submit">Log in</button>
-</form>
+<div class="card">
+    {% if error %}
+    <p class="error">{{ error }}</p>
+    {% endif %}
+    <form method="post" action="/login" class="stacked-form">
+        <input type="hidden" name="csrf_token" value="{{ csrf_token }}">
+        <label for="password">Password</label>
+        <input type="password" id="password" name="password" required autofocus>
+        <button type="submit">Log in</button>
+    </form>
+</div>
 {% endblock %}