fix: remove auto-DDNS registration from installer; default to lan mode

install.sh → make setup was registering 'mycell.pic.ngo' with DDNS at
install time (before the user ever opened the setup wizard). On a fresh
install the user would then open the wizard, choose 'pic1', and get a
401 OTP error because 'mycell' was already registered and the TOTP window
had moved on.

- Remove the register_with_ddns() call from setup_cell.py main(); DDNS
  registration now only happens through the setup wizard
- Change default DOMAIN_MODE from pic_ngo to lan so a bare 'make setup'
  no longer generates an ACME Caddyfile or pre-seeds a pic.ngo identity;
  the wizard collects the real cell name and domain mode from the user

make ddns-register still works for manual / scripted deployments.

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

Makefile

@@ -82,7 +82,7 @@ setup: check-deps
 	@sudo chown -R $${SUDO_USER:-$$(id -un)}:$${SUDO_USER:-$$(id -un)} config/ data/ 2>/dev/null || true
 	CELL_NAME=$(or $(CELL_NAME),mycell) \
 	CELL_DOMAIN=$(or $(CELL_DOMAIN),cell) \
-	DOMAIN_MODE=$(or $(DOMAIN_MODE),pic_ngo) \
+	DOMAIN_MODE=$(or $(DOMAIN_MODE),lan) \
 	CELL_DOMAIN_NAME=$(or $(CELL_DOMAIN_NAME),) \
 	CLOUDFLARE_API_TOKEN=$(or $(CLOUDFLARE_API_TOKEN),) \
 	DUCKDNS_TOKEN=$(or $(DUCKDNS_TOKEN),) \