Add email field to UserContext

Extract email from session cookie and JWT claims. Store email in
cookie data. Needed for coop fragment composition (auth-menu).

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

artdag_common/middleware/auth.py

@@ -21,6 +21,7 @@ class UserContext:
     actor_id: str  # Full actor ID like "@user@server.com"
     token: Optional[str] = None
     l2_server: Optional[str] = None  # L2 server URL for this user
+    email: Optional[str] = None  # User's email address
 
     @property
     def display_name(self) -> str:
@@ -54,6 +55,7 @@ def get_user_from_cookie(request: Request) -> Optional[UserContext]:
             return UserContext(
                 username=username,
                 actor_id=actor_id,
+                email=data.get("email", ""),
             )
         except (json.JSONDecodeError, ValueError, KeyError):
             pass
@@ -71,6 +73,7 @@ def get_user_from_cookie(request: Request) -> Optional[UserContext]:
                 username=username,
                 actor_id=actor_id or "",
                 token=token,
+                email=claims.get("email", ""),
             )
 
     return None
@@ -249,10 +252,13 @@ def set_auth_cookie(response: Any, user: UserContext, max_age: int = 86400 * 30)
         user: User context to store
         max_age: Cookie max age in seconds (default 30 days)
     """
-    data = json.dumps({
+    cookie_data = {
         "username": user.username,
         "actor_id": user.actor_id,
-    })
+    }
+    if user.email:
+        cookie_data["email"] = user.email
+    data = json.dumps(cookie_data)
     cookie_value = base64.b64encode(data.encode()).decode()
 
     response.set_cookie(