fix: inject post data via blueprint context processor

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

bp/market/routes.py

@@ -24,7 +24,9 @@ def register(url_prefix, title):
   async def inject_root():
     market = getattr(g, "market", None)
     market_id = market.id if market else None
+    post_data = getattr(g, "post_data", None) or {}
     return {
+      **post_data,
       "coop_title": market.name if market else title,
       "categories": (await get_nav(g.s, market_id=market_id))["cats"],
       "qs": makeqs_factory()(),