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>

shared/sx/html.py

@@ -206,7 +206,7 @@ def _render(expr: Any, env: dict[str, Any]) -> str:
             return ""
         return _render_list(expr, env)
 
-    # --- dict → skip (data, not renderable) -------------------------------
+    # --- dict → skip (data, not renderable as HTML content) -----------------
     if isinstance(expr, dict):
         return ""
 
@@ -540,6 +540,9 @@ def _render_element(tag: str, args: list, env: dict[str, Any]) -> str:
                 parts.append(f" {attr_name}")
         elif attr_val is True:
             parts.append(f" {attr_name}")
+        elif isinstance(attr_val, dict):
+            from .parser import serialize as _sx_serialize
+            parts.append(f' {attr_name}="{escape_attr(_sx_serialize(attr_val))}"')
         else:
             parts.append(f' {attr_name}="{escape_attr(str(attr_val))}"')
     parts.append(">")