Add nav-tree fragment: blog renders nav for all apps

Blog now provides a nav-tree fragment at /internal/fragments/nav-tree
that accepts app_name and path params for correct aria-selected
highlighting. Blog itself consumes this fragment alongside cart-mini
and auth-menu in a single concurrent fetch_fragments() call.

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

bp/fragments/routes.py

@@ -6,9 +6,10 @@ by other coop apps via the fragment client.
 
 from __future__ import annotations
 
-from quart import Blueprint, Response, request
+from quart import Blueprint, Response, g, render_template, request
 
 from shared.infrastructure.fragments import FRAGMENT_HEADER
+from shared.services.navigation import get_navigation_tree
 
 
 def register():
@@ -30,6 +31,21 @@ def register():
         html = await handler()
         return Response(html, status=200, content_type="text/html")
 
+    # --- nav-tree fragment ---
+    async def _nav_tree_handler():
+        app_name = request.args.get("app_name", "")
+        path = request.args.get("path", "/")
+        first_seg = path.strip("/").split("/")[0]
+        menu_items = await get_navigation_tree(g.s)
+        return await render_template(
+            "fragments/nav_tree.html",
+            menu_items=menu_items,
+            frag_app_name=app_name,
+            frag_first_seg=first_seg,
+        )
+
+    _handlers["nav-tree"] = _nav_tree_handler
+
     # Store handlers dict on blueprint so app code can register handlers
     bp._fragment_handlers = _handlers