fix: config persistence, setup script, and install docs

- app.py: ConfigManager now uses CONFIG_DIR env var for config file path
  instead of hardcoded './config/cell_config.json' — config was being read
  from the image's working directory, making all settings writes ephemeral
  (lost on container restart)
- wireguard_manager: generate_config uses configured address/port instead of
  hardcoded 10.0.0.1 in DNAT rules and Address field
- scripts/setup_cell.py: full setup script — generates WireGuard keys (wg
  binary or Python cryptography fallback), writes wg0.conf and cell_config.json
  with correct _identity key; CELL_NAME / VPN_ADDRESS / WG_PORT env vars
- Makefile: setup target passes env vars through; build-api / build-webui targets
- README: replace install.sh references with make setup && make start

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

api/app.py

@@ -107,7 +107,10 @@ CORS(app)
 app.config['DEVELOPMENT_MODE'] = True  # Set to True for development, False for production
 
 # Initialize enhanced components
-config_manager = ConfigManager(config_file='./config/cell_config.json', data_dir='./data')
+config_manager = ConfigManager(
+    config_file=os.path.join(os.environ.get('CONFIG_DIR', '/app/config'), 'cell_config.json'),
+    data_dir=os.environ.get('DATA_DIR', '/app/data'),
+)
 service_bus = ServiceBus()
 log_manager = LogManager(log_dir='./data/logs')