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/routes/setup.py

@@ -55,11 +55,4 @@ def complete_setup():
     payload = request.get_json(silent=True) or {}
     result = sm.complete_setup(payload)
     status_code = 200 if result.get('success') else 400
-
-    # TODO (Phase 3): if result.get('success') and domain_mode == 'pic_ngo':
-    #     from app import ddns_manager
-    #     name = payload.get('cell_name', '')
-    #     ip   = payload.get('public_ip', '')
-    #     ddns_manager.register(name, ip)
-
     return jsonify(result), status_code