Show the own site icon to Google under every address, and the KANTARCI landing page

- Google kept showing the KantSites K for Elektro Bernhard after the site
  icon was changed. The own icon only existed at 32 and 180 px, and Google
  takes multiples of 48 only; /favicon.svg was a file in public/, so the web
  server always delivered the bundled K.
- The icon is now also cut at 48 and 192 px. The head links /favicon-192.png,
  /favicon.ico serves the 48 px PNG, and /favicon.svg is answered by the
  application: the bundled mark without an own icon (now in
  public/assets/favicon.svg), otherwise a 302 to the 192 px PNG. Sites that
  set their icon earlier get the new sizes on first request. A removed icon
  no longer counts even though its files remain.
- The archive update only adds files to public/, so it now deletes the old
  public/favicon.svg explicitly.
- Seventh local reference site: the KANTARCI Immobilien landing page with a
  three-step rental request, own database, seed and reset scripts.

Tests: 582 PHPUnit on PHP 8.2 and 8.4, 35 node, test:editor and
test:browser green (test:browser reports only the known editor colour
contrast on /grundlagen).

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

scripts/build-kantarci-assets.php

@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * Schneidet aus dem gelieferten Banner (kantarci-banner.jpeg, 901 × 458) die Bilder der
+ * KANTARCI-Landingpage zu: das Logo, das Hausmotiv und ein quadratisches Seitensymbol.
+ * Die Mediathek nimmt kein SVG, deshalb entsteht das Symbol als PNG.
+ *
+ * Aufruf: php scripts/build-kantarci-assets.php <banner.jpeg> <zielordner>
+ */
+
+[$_, $banner, $ziel] = $argv + [null, null, null];
+if (!is_file((string) $banner) || null === $ziel) {
+    fwrite(STDERR, "Aufruf: php scripts/build-kantarci-assets.php <banner.jpeg> <zielordner>\n");
+    exit(1);
+}
+if (!is_dir($ziel)) {
+    mkdir($ziel, 0777, true);
+}
+$quelle = imagecreatefromjpeg($banner);
+
+$zuschnitt = static function (int $x, int $y, int $b, int $h, int $faktor = 1) use ($quelle) {
+    $bild = imagecreatetruecolor($b * $faktor, $h * $faktor);
+    imagecopyresampled($bild, $quelle, 0, 0, $x, $y, $b * $faktor, $h * $faktor, $b, $h);
+
+    return $bild;
+};
+
+// Logo: Bildmarke und Schriftzug oben links, auf Weiß.
+imagepng($zuschnitt(22, 8, 396, 104), $ziel.'/logo-kantarci.png', 9);
+
+// Hausmotiv: der Fotoanteil rechts, oberhalb des goldenen Bands.
+imagejpeg($zuschnitt(500, 0, 401, 334), $ziel.'/haus.jpg', 90);
+
+// Seitensymbol: die Bildmarke mittig auf Weiß, 256 × 256.
+$symbol = imagecreatetruecolor(256, 256);
+imagefill($symbol, 0, 0, imagecolorallocate($symbol, 255, 255, 255));
+imagecopyresampled($symbol, $quelle, 28, 28, 26, 12, 200, 200, 80, 80);
+imagepng($symbol, $ziel.'/symbol-kantarci.png', 9);
+
+echo "Bilder geschrieben nach $ziel\n";