Let a single column sit in a section and give buttons an alignment

A column may now live directly in a section, not only in a row - the way
to centre or narrow a button or a short text without a one-column row.
The align design token applies to buttons; because a button is
inline-flex, auto margins never centred it, so site.css turns an aligned
button into a fit-content block per device. The Drews seed uses the
token for its centred buttons.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/editor-model.test.mjs

@@ -508,7 +508,7 @@ test('the top level takes everything the validator takes there, and nothing else
 // Die Struktur, die der Editor in seinem eigenen Hinweistext beschreibt, gilt jetzt auch fuer
 // die Schaltflaechen: eine Zeile nahm bisher jedes Kind, eine Spalte stand ueberall.
 
-test('a row takes columns and nothing else, a column only lives in a row', () => {
+test('a row takes columns and nothing else, a column lives in a row or alone in a section', () => {
   const document = fixture();
   const rowId = insertRow(document, 3, 's2');
   const row = locate(document.blocks, rowId).block;
@@ -518,8 +518,10 @@ test('a row takes columns and nothing else, a column only lives in a row', () =>
   assert.equal(insertBlockAt(document, 'heading', rowId, 0), null);
   assert.equal(row.children.length, 3);
   assert.ok(insertBlockAt(document, 'heading', row.children[0].id, 0));
-  // Eine Spalte gehoert nicht in eine Sektion.
-  assert.equal(insertBlockAt(document, 'column', 's2', 0), null);
+  // Eine einzelne Spalte darf direkt in einer Sektion stehen - so wird ein Button zentriert -,
+  // aber nicht in einer Spalte oder lose im Dokument.
+  assert.ok(insertBlockAt(document, 'column', 's2', 0));
+  assert.equal(insertBlockAt(document, 'column', row.children[0].id, 0), null);
   assert.ok(insertBlockAt(document, 'column', rowId, 3));
 });