Collabs, as a third configuration of one panel

The same object as a collection and a gallery piece, so it is a config in
subjects.ts and a one-line panel — no new screen.

Two differences, both deliberate. It has no SPECS: a collab is a project rather
than an object, so there is no measurement or raw material and the form is a
name, an address and a description. And its title column is `project_name`,
which is the client's word and stays — the site's API publishes both that and
`title` so this panel reads one field like every other section, and the
translation lives in one router rather than as a branch in every screen.

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

web/sections/PageBuilderPanel.tsx

@@ -24,6 +24,7 @@ const LABELS: Record<string, string> = {
   partners: 'Partners',
   collections: 'Collections',
   pieces: 'Pieces',
+  collabs: 'Collabs',
   'subject-cover': 'Lead photograph',
   'subject-title': 'Name',
   'subject-description': 'Description',
@@ -42,6 +43,7 @@ const PLACES_ONLY: string[] = [
   'partners',
   'collections',
   'pieces',
+  'collabs',
   // The subject blocks hold nothing either: what they draw is the row the URL names.
   'subject-cover',
   'subject-title',