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>

templates/admin/base.html.twig

@@ -18,7 +18,7 @@
        Importzeile sie kennen muss. Sie steht vor jedem Modul-Skript; spaeter geladen wirkt sie
        nicht mehr. #}
     <script type="importmap" nonce="{{ csp_nonce() }}">{"imports":{
-{%- for modul in ['editor-model', 'media', 'design-panel', 'button-design', 'color-control', 'rich-text', 'canvas-overlay', 'library-previews', 'preview-containers', 'page-picker', 'motion', 'slider', 'live-search'] %}
+{%- for modul in ['editor-model', 'media', 'design-panel', 'button-design', 'color-control', 'rich-text', 'canvas-overlay', 'library-previews', 'preview-containers', 'page-picker', 'motion', 'slider', 'live-search', 'icons'] %}
         "/assets/{{ modul }}.js": "/assets/{{ modul }}.js?v={{ kant_version() }}"{{ loop.last ? '' : ',' }}
 {%- endfor %}
     }}</script>