Keep editor open across in-site navigation, retargeting to the new page

load() closed the editor on every navigation, and the close raced the
navigation: the shell's close-time loadPlain re-render resolved last,
swapping the old page back in and replaceStating its URL. Navigation now
leaves the panel open and the existing pagerite:switch-editor retarget
fires while editing; the exception is /_a, where closeEditor({ navigating:
true }) closes the panel without the re-swap or title restore.

frontend/src/EditorShell.vue

@@ -44,11 +44,13 @@ function openShell() {
     editorLang.value = session
   pinPreviewLang()
 }
-function unpinPreviewLang() {
+function unpinPreviewLang(ev) {
   if (!pinned) return
   pinned = false
   setLangOverride(null)
-  loadPlain(currentPath.value)
+  // A close caused by navigation (to /_a) must not re-render the page the
+  // editor was on: the navigation itself is swapping in the target page.
+  if (!ev?.detail?.navigating) loadPlain(currentPath.value)
 }
 watch(editorLang, () => { if (pinned) pinPreviewLang() })
 // The page's primary may be (re)learned while pinned on it (doc accept,