blocks that draw whatever the page is about

`/collections/amadoras-mythology` and `/collections/70s` are the same page with a different subject.
Building each would be absurd; hard-coding the arrangement would put it out of reach of the admin. So
the arrangement is built once — a template page, composed exactly like any other — and every
collection is drawn through it.

Four blocks read the subject: its name, its description, its specs, its photographs. They hold nothing
at all, not even a reference. `collections` holds no content because the list is a query; these hold
none because the subject is the URL, and the same block draws a different collection on the next
request.

Offered only on a template, and HIDDEN rather than disabled elsewhere. A greyed-out row invites "why
not here", and the answer — this page has no subject — is not something a tooltip fixes. Available and
silently drawing nothing is the worse failure.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/sections/PageBuilderPanel.tsx

@@ -1011,7 +1011,13 @@ export const PageBuilderPanel = () => {
         onCreate={(values) => void createSection(values)}
       />
 
-      <AddComponentDialog open={choosing !== null} onCancel={() => setChoosing(null)} onChoose={chooseKind} />
+      <AddComponentDialog
+        open={choosing !== null}
+        onCancel={() => setChoosing(null)}
+        onChoose={chooseKind}
+        // Which decides whether the subject blocks are offered — they render nothing anywhere else.
+        template={page?.is_template}
+      />
 
       <MediaPicker
         open={picking !== null}