fix: inject post data into template context for post header row

The post header bar needs `post` in the template context to render
the page link, feature image, and nav. Added context processor to
spread g.post_data into templates, matching the events app pattern.

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

app.py

@@ -124,6 +124,13 @@ def create_app() -> "Quart":
             abort(404)
         g.market = market
 
+    @app.context_processor
+    async def inject_post():
+        post_data = getattr(g, "post_data", None)
+        if not post_data:
+            return {}
+        return {**post_data}
+
     # --- Root route: market listing ---
     @app.get("/")
     async def markets_listing():