Add auth-menu fragment handler to account app

Phase 2 of fragment composition: account exposes auth-menu fragment
(sign-in button or user email for desktop + mobile) via
/internal/fragments/ endpoint. Updates shared submodule.

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

bp/fragments/routes.py

@@ -2,11 +2,14 @@
 
 Exposes HTML fragments at ``/internal/fragments/<type>`` for consumption
 by other coop apps via the fragment client.
+
+Fragments:
+  auth-menu   Desktop + mobile auth menu (sign-in or user link)
 """
 
 from __future__ import annotations
 
-from quart import Blueprint, Response, request
+from quart import Blueprint, Response, request, render_template
 
 from shared.infrastructure.fragments import FRAGMENT_HEADER
 
@@ -14,7 +17,24 @@ from shared.infrastructure.fragments import FRAGMENT_HEADER
 def register():
     bp = Blueprint("fragments", __name__, url_prefix="/internal/fragments")
 
-    _handlers: dict[str, object] = {}
+    # ---------------------------------------------------------------
+    # Fragment handlers
+    # ---------------------------------------------------------------
+
+    async def _auth_menu():
+        user_email = request.args.get("email", "")
+        return await render_template(
+            "fragments/auth_menu.html",
+            user_email=user_email,
+        )
+
+    _handlers = {
+        "auth-menu": _auth_menu,
+    }
+
+    # ---------------------------------------------------------------
+    # Routing
+    # ---------------------------------------------------------------
 
     @bp.before_request
     async def _require_fragment_header():
@@ -29,6 +49,4 @@ def register():
         html = await handler()
         return Response(html, status=200, content_type="text/html")
 
-    bp._fragment_handlers = _handlers
-
     return bp