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>

public/assets/design-tokens.json

@@ -10,7 +10,8 @@
       "iconcard",
       "stat",
       "quote",
-      "cta"
+      "cta",
+      "button"
     ],
     "values": {
       "left": [