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.
