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/product/routes.py

@@ -157,7 +157,7 @@ def register():
     from models.market import CartItem
     from quart import request, url_for
 
-    @bp.post("/cart")
+    @bp.post("/cart/")
     @clear_cache(tag="browse", tag_scope="user")
     async def cart(slug: str):
         # make sure product exists (we *allow* deleted_at != None later if you want)