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/sxc/examples.sx

@@ -705,7 +705,7 @@
         :sx-get "/examples/api/echo-headers"
         :sx-target "#headers-result"
         :sx-swap "innerHTML"
-        :sx-headers "{\"X-Custom-Token\": \"abc123\", \"X-Request-Source\": \"demo\"}"
+        :sx-headers {:X-Custom-Token "abc123" :X-Request-Source "demo"}
         :class "px-4 py-2 bg-violet-600 text-white rounded hover:bg-violet-700 transition-colors text-sm"
         "Send with headers")
       (div :id "headers-result" :class "p-3 rounded bg-stone-50 text-sm text-stone-400"

sx/sxc/reference.sx

@@ -272,7 +272,7 @@
 (defcomp ~ref-headers-demo ()
   (div :class "space-y-3"
     (button :sx-get "/reference/api/echo-headers"
-      :sx-headers "{\"X-Custom-Token\": \"abc123\", \"X-Request-Source\": \"demo\"}"
+      :sx-headers {:X-Custom-Token "abc123" :X-Request-Source "demo"}
       :sx-target "#ref-headers-result"
       :sx-swap "innerHTML"
       :class "px-4 py-2 bg-violet-600 text-white rounded text-sm hover:bg-violet-700"