Let a button carry a chosen icon before or after its text, and pick icons from tiles

Schema v25: a button takes an optional icon (any name from the shared set or
"none") and an icon position (start or end). Without a choice it keeps the
diagonal arrow behind the text, still with the button-arrow class, so stored
pages and their own CSS look exactly as before.

Thirteen everyday icons join the set (arrow down, chevron, calendar, message,
send, download, document, play, search, heart, info, plus, person). Button,
icon and icon card choose from a tile picker with a drawing per icon, grouped,
keyboard navigable, instead of a select with English keys. icons.js holds the
paths for the editor; a test keeps them equal to the Twig drawings and the
validator list. ADR 108.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

public/assets/editor-model.js

@@ -174,6 +174,7 @@ export function repairSchema(document) {
       if (block.anchor) upgradeDocument(document, 21);
       if ('collapse' in props) upgradeDocument(document, 22);
       if ('faq' in props) upgradeDocument(document, 23);
+      if (block.type === 'button' && ('icon' in props || 'iconPosition' in props)) upgradeDocument(document, 25);
       walk(block.children);
     }
   };
@@ -342,7 +343,7 @@ export const devices = { desktop: 'Desktop', tablet: 'Tablet', mobile: 'Mobil' }
 // blieb bei 12 stehen, während Bausteine schon v13 bis v15 verlangten: in einem aktuellen Dokument
 // liess sich dann gar nichts mehr einfügen, und ein Symbol, ein Slider oder ein externer Inhalt
 // nirgends. Eine Zahl, an einer Stelle.
-export const SCHEMA_VERSION = 24;
+export const SCHEMA_VERSION = 25;
 
 export const VISIBILITY = { all: 'Auf allen Geräten', desktop: 'Nur Desktop', tablet: 'Nur Tablet', mobile: 'Nur Mobil', 'hide-desktop': 'Auf Desktop ausblenden', 'hide-tablet': 'Auf Tablet ausblenden', 'hide-mobile': 'Auf Mobil ausblenden', hidden: 'Überall ausblenden' };