connectors: merge the catalog page into a row-list Connectors page

The standalone Catalog page added nothing the Connectors page could not
do: drop it (component, route, sidebar entry) and move its affordances
onto the Connectors page — the Add-connector dropdown (marketplace /
manual form, now at #connectors/new) and per-row removal for the admin.

Replace the card grid with a sharper row list (4px radius) built for
scanning status and acting; the marketplace's back link now returns to
#connectors. i18n keys renamed catalog.* -> connectors.add/new/*.

web/css/page-shell.css

@@ -82,7 +82,6 @@ plugin-catalog-page,
 plugin-detail-page,
 plugin-page-host,
 marketplace-page,
-catalog-page,
 profile-page {
   display: none; /* toggled by JS */
   flex-direction: column;