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>

templates/admin/theme.html.twig

@@ -162,6 +162,11 @@
                         <input id="base-scale" name="base[headingScale]" type="number" step="0.01" min="1" max="2" value="{{ theme.base.headingScale }}">
                         <p class="field-help">1,25 ergibt eine ruhige Staffelung, 1,5 eine sehr kontrastreiche.</p>
                     </div>
+                    <div>
+                        <label for="base-image-radius">Bildecken</label>
+                        <input id="base-image-radius" name="base[imageRadius]" value="{{ theme.base.imageRadius }}" placeholder="12px">
+                        <p class="field-help">Gilt für jedes Bild auf der Website – auch in Galerien, Logoleisten und Beitragslisten. <code>0px</code> macht sie eckig. Ein einzelnes Bild lässt sich im Editor unter <strong>Design → Bild → Bildecken</strong> abweichend einstellen.</p>
+                    </div>
                     <div>
                         <label for="base-container">Inhaltsbreite</label>
                         <input id="base-container" name="base[containerWidth]" value="{{ theme.base.containerWidth }}" placeholder="1240px">