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/sx/detail.sx

@@ -12,10 +12,13 @@
     (when publish-requested (span :class "inline-block px-3 py-1 rounded-full text-sm font-semibold bg-blue-100 text-blue-800" "Publish requested"))
     edit))
 
+(defcomp ~blog-like-toggle (&key like-url hx-headers heart)
+  (button :sx-post like-url :sx-swap "outerHTML"
+    :sx-headers hx-headers :class "cursor-pointer" heart))
+
 (defcomp ~blog-detail-like (&key like-url hx-headers heart)
   (div :class "absolute top-2 right-2 z-10 text-8xl md:text-6xl"
-    (button :sx-post like-url :sx-swap "outerHTML"
-      :sx-headers hx-headers :class "cursor-pointer" heart)))
+    (~blog-like-toggle :like-url like-url :hx-headers hx-headers :heart heart)))
 
 (defcomp ~blog-detail-excerpt (&key excerpt)
   (div :class "w-full text-center italic text-3xl p-2" excerpt))
@@ -55,8 +58,8 @@
              :like (when has-user
                (~blog-detail-like
                  :like-url like-url
-                 :hx-headers (str "{\"X-CSRFToken\": \"" csrf "\"}")
-                 :heart (if liked "\u2764\ufe0f" "\U0001f90d")))
+                 :hx-headers {:X-CSRFToken csrf}
+                 :heart (if liked "❤️" "🤍")))
              :excerpt (when (not (= custom-excerpt ""))
                (~blog-detail-excerpt :excerpt custom-excerpt))
              :at-bar (~blog-at-bar :tags tags :authors authors)))))