fix: add post header bar to market pages and show market name

- Add post header row to market/index.html template chain
- Fix OOB templates to include post header on HTMX navigation
- Show market name instead of static coop_title in header
- Restore trailing slash on POST /cart/ route; fix templates to
  include trailing slash in cart URLs

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

bp/market/routes.py

@@ -25,7 +25,7 @@ def register(url_prefix, title):
     market = getattr(g, "market", None)
     market_id = market.id if market else None
     return {
-      "coop_title": title,
+      "coop_title": market.name if market else title,
       "categories": (await get_nav(g.s, market_id=market_id))["cats"],
       "qs": makeqs_factory()(),
       "market": market,