Move image corners into the design area

Umut on the DEEG hero image: "It should be rounded, but it was done from
somewhere, and the corners look odd. And the editor has no image corner
rounding at all, does it?"

It did have one - per block, under Design > Bild > Bildecken. What it did not
have was the 12px that enhancements.css set on .block-image for every
installation: rounded on screen, present in no field, and only removable by
overriding it on each block. On a template that is square throughout it shows,
because the elements above the image - the gradient, the sign, the blue call
to action - are square and stick out of the rounded corner.

The radius is now theme.base.imageRadius, validated like the container width,
emitted as --k-image-radius and used by the stylesheet. The default stays 12px
so existing sites look the same; 0px makes every image square. The per-block
setting still wins.

DEEG is set to 0px, in the seed script and in the installation.

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

scripts/seed-immobilien-deeg.php

@@ -283,7 +283,7 @@ $theme->save(
         ['key' => 'body', 'name' => 'Fließtext', 'family' => 'Arial', 'source' => 'system', 'weights' => [400, 600, 700], 'fallback' => 'sans-serif'],
         ['key' => 'heading', 'name' => 'Überschriften', 'family' => 'Arial', 'source' => 'system', 'weights' => [400, 700], 'fallback' => 'sans-serif'],
     ],
-    ['fontFamily' => 'body', 'headingFontFamily' => 'heading', 'fontSize' => '16px', 'lineHeight' => 1.6, 'color' => '@anthrazit', 'backgroundColor' => '@papier', 'linkColor' => '@anthrazit', 'containerWidth' => '1320px', 'headingScale' => 1.25, 'linkUnderline' => false],
+    ['fontFamily' => 'body', 'headingFontFamily' => 'heading', 'fontSize' => '16px', 'lineHeight' => 1.6, 'color' => '@anthrazit', 'backgroundColor' => '@papier', 'linkColor' => '@anthrazit', 'containerWidth' => '1320px', 'headingScale' => 1.25, 'linkUnderline' => false, 'imageRadius' => '0px'],
     ['shape' => 'square', 'appearance' => 'solid', 'size' => 'normal', 'fontFamily' => 'inherit', 'fontSize' => 14, 'fontWeight' => 600, 'letterSpacing' => 0, 'textTransform' => 'none', 'backgroundColor' => '@blau-tief', 'color' => '@papier', 'hoverBackgroundColor' => '@blau-dunkel', 'hoverColor' => '@papier'],
 );
 echo "Farben, Schriften und Buttons gesetzt\n";