Make the start page of a new installation a construction page

Umut: "Kannst du bitte fuer die Erstseite bzw. nach der Installation eine
schoene Erstseite erstellen. Under Construction oder so ohne Details mit melde
dich an usw. Am besten auch noch Logo von KantSites."

starterPage() shipped two sections of instructions for the owner: "Melde dich
an, oeffne sie im Editor ..." and "Die naechsten Schritte" with design, menus
and settings. That stood on the only page a fresh installation serves
publicly, so the visitor read it, not the owner - who gets the same steps on
the last installer screen and in the admin area anyway.

It is now the page such a site actually needs: the KantSites mark, the label
"In Arbeit" in the accent colour, the site name as the single H1, "Hier
entsteht eine neue Webpraesenz.", a short rule and the line "Erstellt mit
KantSites". The section is a flex container with min-height 70vh and a
gradient from @paper to @mint, so the little that is there sits in the middle
of the screen; the colours are palette references and follow a design change.
Everything is an ordinary block with ordinary style values and can be changed
or removed in the editor. The page description is a real sentence now instead
of "NOCH AUSZUFUELLEN", so a freshly installed site does not carry a
placeholder into search results.

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

tests/InstallFlowTest.php

@@ -98,7 +98,7 @@ final class InstallFlowTest extends TestCase
         $home = $this->request('/');
         self::assertSame(200, $home->getStatusCode());
         self::assertStringContainsString('Beispiel GmbH', $home->getContent());
-        self::assertStringContainsString('Diese Seite ist der Anfang.', $home->getContent());
+        self::assertStringContainsString('Hier entsteht eine neue Webpräsenz.', $home->getContent());
         // The public page is still a public page: no script, and the header carries the name.
         self::assertStringContainsString("script-src 'none'", (string) $home->headers->get('Content-Security-Policy'));
         self::assertSame(200, $this->request('/impressum')->getStatusCode());