Fix menu item highlighting with aria-selected attribute

Added app_name Jinja global and aria-selected to nav menu links.
Matches by first path segment (e.g. /market/... → "market") or by
app_name for cross-domain cases (e.g. cart app → "cart" menu item).

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

infrastructure/jinja_setup.py

@@ -37,6 +37,7 @@ def setup_jinja(app: Quart) -> None:
     app.jinja_env.globals["level"] = level
     app.jinja_env.globals["level_up"] = level_up
     app.jinja_env.globals["menu_colour"] = "sky"
+    app.jinja_env.globals["app_name"] = app.name
 
     nav_button = """justify-center cursor-pointer flex flex-row items-center gap-2 rounded bg-stone-200 text-black
       [.hover-capable_&]:hover:bg-yellow-300