feat(post-form): EasyMDE editor + /post-scoped ESM bundle (U3)

Add a page-scoped js/src/post-form.js bundle (built ESM + code-splitting
so U4's heic-to becomes a deferred chunk) loaded only by post-form.html.twig
via a new base head_assets block. EasyMDE replaces the bare textarea with a
minimal toolbar (bold/italic/list/link/preview), FA-free via CSS glyphs, and
syncs codemirror.save() on change + capture-phase submit so the inline
required-field validator and payload see the live value.

Refs R7, R15, KTD2, KTD3.

themes/intotheeast/templates/partials/base.html.twig

@@ -10,6 +10,7 @@
     {% do assets.addJs('theme://js/main.js', {group: 'bottom'}) %}
     {% do assets.addJs('theme://js/nav.js', {group: 'bottom'}) %}
     {% block map_assets %}{% endblock %}
+    {% block head_assets %}{% endblock %}
     {{ assets.css()|raw }}
     {{ assets.js()|raw }}
 </head>