Fix initial sources.manyDecks value to match List type

Changed from Nothing (Maybe) to [] (List) to match the updated
Source.Info type that stores manyDecks as a list of instances.

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

client/src/elm/MassiveDecks.elm

@@ -89,7 +89,7 @@ init flags url key =
             , notifications = Notifications.init
             , remoteMode = remoteMode
             , serverVersion = Nothing
-            , sources = { builtIn = Nothing, manyDecks = Nothing, jsonAgainstHumanity = Nothing }
+            , sources = { builtIn = Nothing, manyDecks = [], jsonAgainstHumanity = Nothing }
             , adverts = { manyDecks = False, atTheParty = False }
             }