installer: restore cell identity prompts and domain setup

Reverts 8d1ef39. The installer must collect cell name, domain mode, and
provider tokens before 'make install' so that DDNS registration,
availability checks, and Caddy TLS can be configured at first boot.

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

api/config_manager.py

@@ -510,6 +510,11 @@ class ConfigManager:
         cfg.setdefault('peer_exit_map', {})
         return dict(cfg)
 
+    def set_ddns_config(self, ddns_cfg: Dict[str, Any]) -> None:
+        """Replace the top-level ddns section and persist."""
+        self.configs['ddns'] = ddns_cfg
+        self._save_all_configs()
+
     def set_connectivity_field(self, field: str, value: Any) -> bool:
         """Set a single field within the connectivity config and persist."""
         cfg = self.configs.setdefault('connectivity', {'exits': {}, 'peer_exit_map': {}})