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/bp/page_admin/routes.py

@@ -0,0 +1,25 @@
+from __future__ import annotations
+
+from quart import make_response, Blueprint
+
+from shared.browser.app.authz import require_admin
+from shared.browser.app.utils.htmx import is_htmx_request
+
+
+def register():
+    bp = Blueprint("page_admin", __name__)
+
+    @bp.get("/")
+    @require_admin
+    async def admin(**kwargs):
+        from shared.sexp.page import get_template_context
+        from sexp.sexp_components import render_page_admin_page, render_page_admin_oob
+
+        tctx = await get_template_context()
+        if not is_htmx_request():
+            html = await render_page_admin_page(tctx)
+        else:
+            html = await render_page_admin_oob(tctx)
+        return await make_response(html)
+
+    return bp