Add a Before launch placeholder check to admin

Lists pages still showing built-in wording, notes holding placeholder
text, unpublished posts and live notes missing the field their page is
built around, each linked to its editor.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012aSbo2D1qP8cMFQCykf7kx

services/admin/src/layout.ts

@@ -9,6 +9,7 @@ export interface Flash {
 const nav: [string, string][] = [
   ["/", "Overview"],
   ["/site", "Site pages"],
+  ["/launch", "Before launch"],
   ["/review", "Review"],
   ["/inbox", "Inbox"],
   ["/notes", "Notes"],