Fix DDNS registration and wizard pre-fill after installer run

DDNS registration (setup_cell.py):
- Replace pyotp dependency with stdlib TOTP (HMAC-SHA1, RFC 6238)
  pyotp is only available inside the Docker container, not on the host
  where setup_cell.py runs — registration was silently skipped every time
- OTP header still sent if generation succeeds; omitted gracefully if not

Wizard pre-fill (setup_manager + Setup.jsx):
- GET /api/setup/status now returns 'preconfigured' dict with cell_name,
  domain_mode, domain_name, and provider tokens from installer-written config
- Setup.jsx fetches status on mount and pre-fills all form state so the
  user only needs to set password, services, and timezone — not re-enter
  the identity they already configured in the bash installer
- Fails silently so wizard still works on fresh installs with no config

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

tests/test_setup_manager.py

@@ -300,3 +300,23 @@ def test_get_setup_status_timezones_match_module_constant(setup_manager, mock_co
     mock_config_manager.get_identity.return_value = {}
     status = setup_manager.get_setup_status()
     assert status['available_timezones'] == AVAILABLE_TIMEZONES
+
+
+def test_get_setup_status_preconfigured_empty_when_identity_blank(setup_manager, mock_config_manager):
+    mock_config_manager.get_identity.return_value = {}
+    status = setup_manager.get_setup_status()
+    assert status['preconfigured'] == {}
+
+
+def test_get_setup_status_preconfigured_returns_installer_values(setup_manager, mock_config_manager):
+    mock_config_manager.get_identity.return_value = {
+        'cell_name': 'myhome',
+        'domain_mode': 'pic_ngo',
+        'domain_name': 'myhome.pic.ngo',
+    }
+    status = setup_manager.get_setup_status()
+    pre = status['preconfigured']
+    assert pre['cell_name'] == 'myhome'
+    assert pre['domain_mode'] == 'pic_ngo'
+    assert pre['domain_name'] == 'myhome.pic.ngo'
+    assert 'cloudflare_api_token' not in pre