feat(i18n): share lobby message catalog across frontend/backend

shared/i18n/lobby.json

@@ -0,0 +1,27 @@
+{
+  "frontend": {
+    "errors": {
+      "session_code_required": "Session code is required.",
+      "session_fetch_failed": "Could not load lobby status.",
+      "join_failed": "Join failed. Check code or nickname and try again.",
+      "start_round_failed": "Could not start round. Refresh the lobby and try again.",
+      "session_not_found": "Session code is invalid or the session no longer exists.",
+      "nickname_invalid": "Nickname must be between 2 and 40 characters.",
+      "nickname_taken": "Nickname is already taken.",
+      "unknown": "Action failed. Refresh status and try again."
+    }
+  },
+  "backend": {
+    "error_codes": {
+      "session_code_required": "session_code_required",
+      "nickname_invalid": "nickname_invalid",
+      "session_not_found": "session_not_found",
+      "session_not_joinable": "session_not_joinable",
+      "nickname_taken": "nickname_taken",
+      "category_slug_required": "category_slug_required",
+      "category_not_found": "category_not_found",
+      "round_start_invalid_phase": "round_start_invalid_phase",
+      "round_already_configured": "round_already_configured"
+    }
+  }
+}