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>

events/sexp/sexp_components.py

@@ -15,6 +15,7 @@ from shared.sexp.jinja_bridge import render, load_service_components
 from shared.sexp.helpers import (
     call_url, get_asset_url, root_header_html,
     post_header_html as _shared_post_header_html,
+    post_admin_header_html,
     oob_header_html,
     search_mobile_html, search_desktop_html,
     full_page, oob_page,
@@ -1352,11 +1353,19 @@ async def render_day_admin_oob(ctx: dict) -> str:
 # Calendar admin
 # ---------------------------------------------------------------------------
 
+def _events_post_admin_header_html(ctx: dict, *, oob: bool = False,
+                                    selected: str = "") -> str:
+    """Post-level admin row for events — delegates to shared helper."""
+    slug = (ctx.get("post") or {}).get("slug", "")
+    return post_admin_header_html(ctx, slug, oob=oob, selected=selected)
+
+
 async def render_calendar_admin_page(ctx: dict) -> str:
     """Full page: calendar admin."""
     content = _calendar_admin_main_panel_html(ctx)
     hdr = root_header_html(ctx)
     child = (_post_header_html(ctx)
+             + _events_post_admin_header_html(ctx, selected="calendars")
              + _calendar_header_html(ctx) + _calendar_admin_header_html(ctx))
     hdr += render("header-child", inner_html=child)
     return full_page(ctx, header_rows_html=hdr, content_html=content)
@@ -1365,7 +1374,8 @@ async def render_calendar_admin_page(ctx: dict) -> str:
 async def render_calendar_admin_oob(ctx: dict) -> str:
     """OOB response: calendar admin."""
     content = _calendar_admin_main_panel_html(ctx)
-    oobs = _calendar_header_html(ctx, oob=True)
+    oobs = (_events_post_admin_header_html(ctx, oob=True, selected="calendars")
+            + _calendar_header_html(ctx, oob=True))
     oobs += _oob_header_html("calendar-header-child", "calendar-admin-header-child",
                               _calendar_admin_header_html(ctx))
     return oob_page(ctx, oobs_html=oobs, content_html=content)
@@ -1383,6 +1393,7 @@ async def render_slots_page(ctx: dict) -> str:
     content = render_slots_table(slots, calendar)
     hdr = root_header_html(ctx)
     child = (_post_header_html(ctx)
+             + _events_post_admin_header_html(ctx, selected="calendars")
              + _calendar_header_html(ctx) + _calendar_admin_header_html(ctx))
     hdr += render("header-child", inner_html=child)
     return full_page(ctx, header_rows_html=hdr, content_html=content)
@@ -1393,7 +1404,8 @@ async def render_slots_oob(ctx: dict) -> str:
     slots = ctx.get("slots") or []
     calendar = ctx.get("calendar")
     content = render_slots_table(slots, calendar)
-    oobs = _calendar_admin_header_html(ctx, oob=True)
+    oobs = (_events_post_admin_header_html(ctx, oob=True, selected="calendars")
+            + _calendar_admin_header_html(ctx, oob=True))
     return oob_page(ctx, oobs_html=oobs, content_html=content)