fix: binary asset serving + checkout centering

el_runtime: http_response() JSON-encoded the body via jb_emit_escaped(),
which stops at the first null byte. PNG/binary files contain null bytes
at byte 8 (IHDR chunk length), so only 8 bytes were served — browsers
received a corrupt/truncated image and showed broken icons.

Fix: when _tl_fs_read_len > 0 (binary fs_read), copy raw bytes into a
thread-local side-channel (_tl_binary_body/_tl_binary_size) and write
the sentinel "__el_binary__" into the envelope body field. http_send_response()
detects the sentinel and substitutes the real bytes for sending.

checkout.el: .checkout-shell, .checkout-summary, and .checkout-form-wrap
had no CSS, leaving the page left-aligned and single-column. Added grid
layout (2-col desktop, 1-col mobile), max-width centering, and sticky
order summary.

src/checkout.el

@@ -345,7 +345,31 @@ fn checkout_page(plan: String, pub_key: String) -> String {
 }
 
 fn checkout_style_html() -> String {
-    let css: String = ".checkout-plan-name {
+    let css: String = ".checkout-shell {
+  max-width: 980px;
+  margin: 0 auto;
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 4rem;
+  align-items: start;
+}
+.checkout-summary {
+  position: sticky;
+  top: 2rem;
+}
+.checkout-form-wrap {
+  min-width: 0;
+}
+@media (max-width: 860px) {
+  .checkout-shell {
+    grid-template-columns: 1fr;
+    gap: 2.5rem;
+  }
+  .checkout-summary {
+    position: static;
+  }
+}
+.checkout-plan-name {
   font-family: var(--head);
   font-size: clamp(1.5rem, 3vw, 2rem);
   font-weight: 600;