Add select_colours as Jinja global for consistent nav highlighting

select_colours was only defined via {% set %} in the root header
template, making it undefined in market/browse nav templates. Moving
it to a Jinja global ensures aria-selected styling works everywhere.

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

infrastructure/jinja_setup.py

@@ -39,10 +39,14 @@ def setup_jinja(app: Quart) -> None:
     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
+    select_colours = """
       [.hover-capable_&]:hover:bg-yellow-300
       aria-selected:bg-stone-500 aria-selected:text-white
       [.hover-capable_&[aria-selected=true]:hover]:bg-orange-500"""
+    app.jinja_env.globals["select_colours"] = select_colours
+
+    nav_button = f"""justify-center cursor-pointer flex flex-row items-center gap-2 rounded bg-stone-200 text-black
+      {select_colours}"""
 
     styles = {
         "pill": """