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>

scripts/setup_cell.py

@@ -304,25 +304,28 @@ def register_with_ddns(cell_name: str) -> None:
         print(f'[WARN]    Could not detect public IP: {e} — skipping DDNS registration')
         return
 
-    # Generate TOTP code (requires pyotp; if not available fall back gracefully)
+    # Generate TOTP using stdlib only — no third-party package needed
+    otp = ''
     try:
-        import pyotp
-        otp = pyotp.TOTP(DDNS_TOTP_SECRET).now()
-    except ImportError:
-        # Try python3 -c as a subprocess fallback
-        try:
-            otp = subprocess.check_output(
-                ['python3', '-c', f"import pyotp; print(pyotp.TOTP('{DDNS_TOTP_SECRET}').now())"]
-            ).decode().strip()
-        except Exception as e:
-            print(f'[WARN]    pyotp not available and fallback failed: {e} — skipping DDNS')
-            return
+        import base64 as _b64, hashlib as _hl, hmac as _hmac, struct as _struct
+        import time as _time
+        _key = _b64.b32decode(DDNS_TOTP_SECRET.upper())
+        _t = int(_time.time()) // 30
+        _h = _hmac.new(_key, _struct.pack('>Q', _t), _hl.sha1).digest()
+        _offset = _h[-1] & 0xF
+        _code = _struct.unpack('>I', _h[_offset:_offset + 4])[0] & 0x7FFFFFFF
+        otp = f'{_code % 1_000_000:06d}'
+    except Exception as e:
+        print(f'[WARN]    Could not generate OTP: {e} — registering without OTP header')
 
     data = json.dumps({'name': cell_name, 'ip': public_ip}).encode()
+    headers = {'Content-Type': 'application/json'}
+    if otp:
+        headers['X-Register-OTP'] = otp
     req = urllib.request.Request(
         f'{DDNS_URL}/register',
         data=data,
-        headers={'Content-Type': 'application/json', 'X-Register-OTP': otp},
+        headers=headers,
         method='POST',
     )
     try: