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>

shared/static/scripts/sx-editor.js

@@ -2249,7 +2249,9 @@
       return null;
     }
 
-    root.className = (root.className || "") + " sx-editor";
+    // Clear any previous mount
+    root.innerHTML = "";
+    root.className = (root.className || "").replace(/\bsx-editor\b/g, "").trim() + " sx-editor";
 
     var container = el("div", { className: "sx-blocks-container" });
     root.appendChild(container);