web + ui: Pass init cookie from server to client

ui/src/Main.elm

@@ -2,7 +2,8 @@ module Main exposing (main)
 
 import Browser exposing (Document, UrlRequest)
 import Browser.Navigation as Nav
-import Data.Session as Session exposing (Session, decoder)
+import Data.AppConfig as AppConfig exposing (AppConfig)
+import Data.Session as Session exposing (Session)
 import Html exposing (..)
 import Json.Decode as D exposing (Value)
 import Page.Home as Home
@@ -34,11 +35,27 @@ type alias Model =
     }
 
 
+type alias InitConfig =
+    { appConfig : AppConfig
+    , session : Session.Persistent
+    }
+
+
 init : Value -> Url -> Nav.Key -> ( Model, Cmd Msg )
-init sessionValue location key =
+init configValue location key =
     let
+        configDecoder =
+            D.map2 InitConfig
+                (D.field "app-config" AppConfig.decoder)
+                (D.field "session" Session.decoder)
+
         session =
-            Session.init key location (Session.decodeValueWithDefault sessionValue)
+            case D.decodeValue configDecoder configValue of
+                Ok config ->
+                    Session.init key location config.session
+
+                Err error ->
+                    Session.initError key location (D.errorToString error)
 
         ( subModel, _ ) =
             Home.init session