Version static URLs by mtime; flatter tabs

The stylesheet and bundle were served with Last-Modified only, so a browser
could pair new JavaScript with a stale app.css and render the tab strip and
thumbnails unstyled. Their URLs now carry the file's mtime. Tabs are flat on
the chrome with the open one underlined; the close button shows on hover.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

clips/templates/clips/index.html

@@ -20,12 +20,12 @@ and PUTs with ordinary CSRF protection — no endpoint in this app is exempt.
     <meta charset="utf-8">
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <title>arthur</title>
-    <link rel="stylesheet" href="{% static 'arthur/app.css' %}">
+    <link rel="stylesheet" href="{% static 'arthur/app.css' %}?v={{ css_version }}">
   </head>
   <body>
     {% csrf_token %}
     <div id="app"></div>
     <script src="{% static 'mediapipe/vision_bundle.js' %}"></script>
-    <script src="{% static 'arthur/js/main.js' %}"></script>
+    <script src="{% static 'arthur/js/main.js' %}?v={{ js_version }}"></script>
   </body>
 </html>