fix(site-builder): serialise all saves; honest pill for conflicts and new sites

A manual Save during Publish sent a second save with the same base_rev; the
loser 409'd against its own tab (and could abort the publish).

- useWhpApi: every save runs through one in-flight promise (SaveGuard
  saveInFlight). A save requested while one is out waits, then sends only
  if still dirty, with the rev the first returned; payload is read from the
  editor at send time. Forced (empty/overwrite) saves always send.
- Save buttons are disabled while publishing.
- Pill: a conflict reads 'Unsaved changes' (modal open) from Save and from
  Publish alike; a never-saved site (load rev null) reads 'Not saved yet'.
- Autosave interval reads runSave through a ref, so editing can't keep
  restarting it and starve autosave.
- Fix stale 'Throws like exportBodyHtml' comment.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

craft/src/styles/editor.css

@@ -1992,6 +1992,7 @@ body {
 .save-pill.saved { background: rgba(16, 185, 129, 0.12); color: #10b981; border-color: rgba(16, 185, 129, 0.25); }
 .save-pill.published { background: rgba(16, 185, 129, 0.2); color: #34d399; border-color: rgba(16, 185, 129, 0.4); }
 .save-pill.failed { background: rgba(239, 68, 68, 0.15); color: #f87171; border-color: rgba(239, 68, 68, 0.4); }
+.save-pill.new { background: rgba(161, 161, 170, 0.12); color: #a1a1aa; border-color: rgba(161, 161, 170, 0.3); }
 .topbar-mobile .save-pill { padding: 3px 6px; font-size: 10px; }
 
 /* Error toast: fixed, stays until dismissed */