Fix duplicate headers on HTMX nav, editor content loading, and double mount

- Nest admin header inside post-header-child (layouts.py/helpers.py) so
  full-page DOM matches OOB swap structure, eliminating duplicate headers
- Clear post-header-child on post layout OOB to remove stale admin rows
- Read SX initial content from #sx-content-input instead of
  window.__SX_INITIAL__ to avoid escaping issues through SX pipeline
- Fix client-side SX parser RE_STRING to handle escaped newlines
- Clear root element in SxEditor.mount() to prevent double content on
  HTMX re-mount
- Remove unused ~blog-editor-sx-initial component

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

blog/sx/editor.sx

@@ -153,10 +153,6 @@
     "  sync();"
     "})();"))
 
-;; SX initial content script for edit form
-(defcomp ~blog-editor-sx-initial (&key sx-content)
-  (script (str "window.__SX_INITIAL__ = '" sx-content "' || null;")))
-
 (defcomp ~blog-editor-styles (&key css-href)
   (<> (link :rel "stylesheet" :href css-href)
     (style

blog/sx/sx_components.py

@@ -1271,7 +1271,7 @@ def render_editor_panel(save_error: str | None = None, is_page: bool = False) ->
         "\n"
         "    if (typeof SxEditor !== 'undefined') {\n"
         "      SxEditor.mount('sx-editor', {\n"
-        "        initialSx: window.__SX_INITIAL__ || null,\n"
+        "        initialSx: (document.getElementById('sx-content-input') || {}).value || null,\n"
         "        csrfToken: csrfToken,\n"
         "        uploadUrls: uploadUrls,\n"
         f"        oembedUrl: '{oembed_url}',\n"
@@ -1748,16 +1748,13 @@ def _post_edit_content_sx(ctx: dict) -> str:
         badge_parts.append(f'(span :class "inline-block px-2 py-0.5 rounded-full text-xs font-semibold bg-green-100 text-green-800" "Emailed{suffix}")')
     footer_extra_sx = SxExpr("(<> " + " ".join(badge_parts) + ")") if badge_parts else None
 
-    # Escape sx_content for JS string literal
-    sx_initial_escaped = sx_content.replace('\\', '\\\\').replace("'", "\\'").replace('\n', '\\n').replace('\r', '')
-
     parts: list[str] = []
 
     # Error banner
     if save_error:
         parts.append(sx_call("blog-editor-error", error=save_error))
 
-    # Form
+    # Form (sx_content_val populates #sx-content-input; JS reads from there)
     parts.append(sx_call("blog-editor-edit-form",
         csrf=csrf,
         updated_at=str(updated_at),
@@ -1778,9 +1775,6 @@ def _post_edit_content_sx(ctx: dict) -> str:
     # Publish-mode JS
     parts.append(sx_call("blog-editor-publish-js", already_emailed=already_emailed))
 
-    # SX initial content
-    parts.append(sx_call("blog-editor-sx-initial", sx_content=sx_initial_escaped))
-
     # Editor CSS + styles
     parts.append(sx_call("blog-editor-styles", css_href=editor_css))
     parts.append(sx_call("sx-editor-styles"))
@@ -1863,7 +1857,7 @@ def _post_edit_content_sx(ctx: dict) -> str:
         '    });'
         "    if (typeof SxEditor !== 'undefined') {"
         "      SxEditor.mount('sx-editor', {"
-        "        initialSx: window.__SX_INITIAL__ || null,"
+        "        initialSx: (document.getElementById('sx-content-input') || {}).value || null,"
         '        csrfToken: csrfToken,'
         '        uploadUrls: uploadUrls,'
         f"        oembedUrl: '{oembed_url}',"