Resume interrupted editor co-op runs

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lang/en.lang

@@ -664,6 +664,7 @@ net.coop_builtin = Built-in campaign
 net.coop_league = League against the Lightwatch
 net.coop_solo = AI partner
 net.coop_solo_started = Co-op without the server: your AI partner builds its own dungeon, follows your attack flag and helps out with gold.
+net.coop_resume = Continue M{0}
 net.coop_editor_mission = Editor campaign {0}: mission {1} ({2})
 net.coop_editor_none = No mission of the editor campaign fits co-op (map at least 72x72, a goal besides rival masters)
 net.join = Join