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>

sx/sx/examples-content.sx

@@ -277,7 +277,7 @@
     :description "sx-vals adds extra key/value pairs to the request parameters. sx-headers adds custom HTTP headers. The server echoes back what it received."
     :demo-description "Click each button to see what the server receives."
     :demo (~vals-headers-demo)
-    :sx-code ";; Send extra values with the request\n(button\n  :sx-get \"/examples/api/echo-vals\"\n  :sx-vals \"{\\\"source\\\": \\\"button\\\"}\"\n  \"Send with vals\")\n\n;; Send custom headers\n(button\n  :sx-get \"/examples/api/echo-headers\"\n  :sx-headers \"{\\\"X-Custom-Token\\\": \\\"abc123\\\"}\"\n  \"Send with headers\")"
+    :sx-code ";; Send extra values with the request\n(button\n  :sx-get \"/examples/api/echo-vals\"\n  :sx-vals \"{\\\"source\\\": \\\"button\\\"}\"\n  \"Send with vals\")\n\n;; Send custom headers\n(button\n  :sx-get \"/examples/api/echo-headers\"\n  :sx-headers {:X-Custom-Token \"abc123\"}\n  \"Send with headers\")"
     :handler-code "@bp.get(\"/examples/api/echo-vals\")\nasync def api_echo_vals():\n    vals = dict(request.args)\n    return sx_response(\n        f'(~echo-result :label \"values\" :items (...))')\n\n@bp.get(\"/examples/api/echo-headers\")\nasync def api_echo_headers():\n    custom = {k: v for k, v in request.headers\n              if k.startswith(\"X-\")}\n    return sx_response(\n        f'(~echo-result :label \"headers\" :items (...))')"
     :comp-placeholder-id "vals-comp"
     :wire-placeholder-id "vals-wire"))