Show a live preview of the header on the Design tab's Menu card

The menu options had grown to a dozen switches with nothing to look at
but the blog after a save. A new POST /b/{sub}/design/preview reads the
form as a save would and renders the header alone — the real blog.css
and theme CSS, the modules in their order, the menu as edited — into a
sandboxed frame that refreshes shortly after every change, with a Wide
screen / Phone toggle. Nothing is stored.

The frame keeps the dashboard's origin (allow-same-origin, no scripts)
so the blog's logo and fonts, linked through /b/<sub>/media since the
root host's /media is the root blog's, get the session cookie; that is
safe because no header module is owner HTML. The frame runs no scripts,
so the page's own script folds the menu in it the way the blog does.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sd8UPWrvyYCLj97JexNw3A

1 files changed, 2 insertions, 1 deletions

diff --git a/README.md b/README.md
index 7358e8d..120a9a6 100644
--- a/README.md
+++ b/README.md
@@ -39,7 +39,8 @@ Server-rendered HTML, no JavaScript required, works on old browsers and phones.
   links can be bold, uppercase, underlined always, on hover or never, small or large,
   tight or wide apart, in any combination; the current page's link is underlined, bold,
   boxed or left alone, and the header menu sits left, centred, right or spread across the bar —
-  or folds into a ☰ button (beside the title or logo above it) when its links do not fit. Posts per
+  or folds into a ☰ button (beside the title or logo above it) when its links do not fit.
+  A live preview of the header shows the menu as set, on a wide screen or a phone. Posts per
   page, the date format, the site title size and the footer alignment live here too,
   as does whether listings show whole posts or **summaries** with a "Read more" link
   (a post's summary ends at a `<!--more-->` line, or after its first paragraphs).