panel: stack Add an application above the list, drop the two-column row

The side-by-side .split row read badly: main is capped at 48rem, so the
applications table had to live in roughly 27rem and its actions column
squeezed four controls into it.

Put the create form directly above the list instead — the order the
domains page already uses for "Add a sending domain" above "Domains" —
and delete .split, which nothing else used. The empty-state text follows
the same page's wording.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -21,9 +21,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.1.0/); version
   They are all buttons now: filled for a card's own action, compact and
   outlined where actions cluster in a table row or the nav bar. An `<a>` is
   once again only used for navigation.
-- panel: on the domain page, **Applications** and **Add an application** sit
-  side by side on a wide viewport instead of the form being stranded below the
-  domain rate limit; they stack again on a narrow one.
+- panel: on the domain page **Add an application** now sits directly above the
+  **Applications** list — the same order the domains page uses for its own add
+  form — instead of being stranded below the domain rate limit.
 - ci: hermetic container e2e suite (`test/e2e`, a separate Go module) gates
   image publishing — `make e2e` locally, and `go test ./...` in `test/e2e` as
   a required step in `release.yml` before a version tag's image is pushed.