perf(overlay): only ship editor assets to pages that mount an editor

Addresses Important #2 from the final code review.

The asset partial was previously included unconditionally for any
overlay detail page. Workshop overlays and read-only files-type
overlays (when the viewer isn't the owner or admin) have no
data-editor-language textarea, so the ~30 KB of Prism + CodeJar + JS
+ CSS shipped pointlessly. Gate the include on the two cases that
actually mount an editor: script-type overlays (bash editor) and
files-type overlays where the current user has edit rights (the
files-editor modal).

I-1 from the review (race window during "Loading…") was confirmed
moot — editorDialog.showModal() only fires after the fetch resolves
(files-overlay.js:409), so the dialog is invisible during the
fetch-and-placeholder window and the user can't type into it.

I-3 (Playwright coverage of the language dropdown override) is real
follow-up work that needs a new files-type overlay seed in the
live_server fixture. Deferred as a v2 ticket.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/l4d2web/templates/overlay_detail.html

@@ -282,5 +282,11 @@
 
 <script src="{{ url_for('static', filename='js/files-overlay.js') }}" defer></script>
 {% endif %}
-{% include '_editor_assets.html' %}
+{# Only include the ~30 KB of editor assets on pages that actually mount
+   an editor: script-type overlays (bash editor) and files-type overlays
+   that the current user can edit (the files-editor modal). Workshop
+   overlays and read-only pages skip the include entirely. #}
+{% if overlay.type == 'script' or files_can_edit %}
+  {% include '_editor_assets.html' %}
+{% endif %}
 {% endblock %}