Replace JSON sx-headers with SX dict expressions, fix blog like component

sx-headers attributes now use native SX dict format {:key val} instead of
JSON strings. Eliminates manual JSON string construction in both .sx files
and Python callers.

- sx.js: parse sx-headers/sx-vals as SX dict ({: prefix) with JSON fallback,
  add _serializeDict for dict→attribute serialization, fix verbInfo scope in
  _doFetch error handler
- html.py: serialize dict attribute values via SX serialize() not str()
- All .sx files: {:X-CSRFToken csrf} replaces (str "{\"X-CSRFToken\": ...}")
- All Python callers: {"X-CSRFToken": csrf} dict replaces f-string JSON
- Blog like: extract ~blog-like-toggle, fix POST returning wrong component,
  fix emoji escapes in .sx (parser has no \U support), fix card :hx-headers
  keyword mismatch, wrap sx_content in SxExpr for evaluation

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

blog/services/blog_page.py

@@ -1,6 +1,13 @@
 """Blog page data service — provides serialized dicts for .sx defpages."""
 from __future__ import annotations
 
+from shared.sx.parser import SxExpr
+
+
+def _sx_content_expr(raw: str) -> SxExpr | None:
+    """Wrap non-empty sx_content as SxExpr so it serializes unquoted."""
+    return SxExpr(raw) if raw else None
+
 
 class BlogPageService:
     """Service for blog page data, callable via (service "blog-page" ...)."""
@@ -424,7 +431,7 @@ class BlogPageService:
             "authors": authors,
             "feature_image": post.get("feature_image"),
             "html_content": post.get("html", ""),
-            "sx_content": post.get("sx_content", ""),
+            "sx_content": _sx_content_expr(post.get("sx_content", "")),
         }
 
     async def preview_data(self, session, *, slug=None, **kw):