Unify post admin nav across all services

Move post admin header into shared/sexp/helpers.py so blog, cart,
events, and market all render the same admin row with identical nav:
calendars | markets | payments | entries | data | edit | settings.

All links are external (cross-service). The selected item shows
highlighted on the right and as white text next to "admin" on the left.

- blog: delegates to shared helper, removes blog-specific nav builder
- cart: delegates to shared helper for payments admin
- events: adds shared admin row (selected=calendars) to calendar admin
- market: adds /<slug>/admin/ route + page_admin blueprint, delegates
  to shared helper (selected=markets). Fixes 404 on page-level admin.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

market/app.py

@@ -11,7 +11,7 @@ from sqlalchemy import select
 from shared.infrastructure.factory import create_base_app
 from shared.config import config
 
-from bp import register_market_bp, register_all_markets, register_page_markets, register_fragments, register_actions, register_data
+from bp import register_market_bp, register_all_markets, register_page_markets, register_page_admin, register_fragments, register_actions, register_data
 
 
 async def market_context() -> dict:
@@ -111,6 +111,12 @@ def create_app() -> "Quart":
         url_prefix="/<slug>",
     )
 
+    # Page admin: /<slug>/admin/ — post-level admin for markets
+    app.register_blueprint(
+        register_page_admin(),
+        url_prefix="/<slug>/admin",
+    )
+
     # Market blueprint nested under post slug: /<page_slug>/<market_slug>/
     app.register_blueprint(
         register_market_bp(