Fix DHCP and service startup issues

Fixed three critical issues preventing DHCP from working:

1. Config file rendering: Placeholders (@GATEWAY_IP@, etc.) weren't being
   substituted. Using direct heredoc writes instead of sed for reliability.

2. Service dependency blocking: van-br0-setup was blocked waiting for
   hostapd.service to start, preventing bridge members from being added.
   Changed to only depend on systemd-networkd so boot completes.

3. Port 53 conflict: systemd-resolved was listening on port 53, blocking
   dnsmasq from binding. Disabled DNS in systemd-resolved config.

R5C now operational:
- 5GHz AP on channel 149
- Bridge: 10.42.1.1/24 with enP2p33s0 + wlp1s0
- DHCP: 10.42.1.10-254 (4+ active leases)
- van-ap-dnsmasq service: running and serving DHCP
- hostapd-2g: disabled (interface wlx3c3332002066 not available)

Disabled hostapd-2g.service until D-Link RTL8188GU is fixed (stuck in CDROM mode).
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

deploy.conf

@@ -17,7 +17,7 @@ SKIP_COCKPIT=1
 WIFI_5G_IFACE=wlp1s0
 
 # 2.4GHz AP radio (currently: same as 5GHz on Armbian R5C; on Debian was D-Link DWA-171)
-WIFI_2G_IFACE=wlp1s0
+WIFI_2G_IFACE=wlx3c3332002066
 
 # Wired LAN port, USB-attached gigabit adapter (RTL8153-family)
 LAN_USB_IFACE=enP2p33s0