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/views.py

@@ -119,10 +119,27 @@ def _crop_blob(chunks):
 # the page
 
 
+def _asset_version(relative):
+    """A static file's modification time, for its URL.
+
+    The stylesheet and the bundle are served with `Last-Modified` and nothing
+    else, so a browser is free to keep a stale copy on heuristic freshness — and
+    new JavaScript over an old stylesheet renders a pane the stylesheet has never
+    heard of as bare elements. A version in the URL makes each edit a new URL."""
+    for root in settings.STATICFILES_DIRS:
+        path = Path(root) / relative
+        if path.exists():
+            return str(int(path.stat().st_mtime))
+    return "0"
+
+
 def page(request):
     """The host page. This replaced `frontend/public/index.html` at step 9, and
     `:dev-http` in shadow-cljs.edn went away with it."""
-    return render(request, "clips/index.html")
+    return render(request, "clips/index.html", {
+        "css_version": _asset_version("arthur/app.css"),
+        "js_version": _asset_version("arthur/js/main.js"),
+    })
 
 
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