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/js/src/post-form.css

@@ -0,0 +1,21 @@
+/*
+ * /post form bundle CSS. U3 scope: make the EasyMDE editor usable without the
+ * FontAwesome dependency (toolbar glyphs supplied here). Field Notes theming,
+ * mobile layout, and the "More options" disclosure land in U5.
+ */
+
+/* EasyMDE toolbar glyphs — replace the FontAwesome icons EasyMDE expects. */
+.editor-toolbar .mde-btn::before {
+    font-style: normal;
+    font-weight: 400;
+    font-family: inherit;
+}
+.editor-toolbar .mde-bold::before    { content: 'B'; font-weight: 700; }
+.editor-toolbar .mde-italic::before  { content: 'I'; font-style: italic; }
+.editor-toolbar .mde-ul::before      { content: '\2022\2002\2014'; } /* •  — */
+.editor-toolbar .mde-link::before    { content: '\1F517'; }          /* 🔗 */
+.editor-toolbar .mde-preview::before { content: '\1F441'; }          /* 👁 */
+
+.EasyMDEContainer .CodeMirror {
+    min-height: 180px;
+}