Fix WiFi conflict and SSH accessibility on fresh flash

- Remove wpa_supplicant@wlan0.service enablement: Netplan spawns its own
  wpa_supplicant instance, enabling the @wlan0 unit creates a second one
  that fights for the socket, causing WiFi instability and SSH drops
- Write /etc/nftables.conf to card at flash time: Armbian bookworm default
  nftables policy drops all input; SSH was unreachable despite sshd running.
  Ruleset allows established, loopback, ICMP, and TCP/22 only

Fixes DevTrack #317 and #295

operator_setup.sh

@@ -525,15 +525,6 @@ DHCP=yes
 IPv6PrivacyExtensions=yes
 EOF
 
-    # Enable wpa_supplicant@wlan0 if the template service exists
-    WPA_TEMPLATE="${MOUNT_POINT}/lib/systemd/system/wpa_supplicant@.service"
-    WPA_WANTS="${MOUNT_POINT}/etc/systemd/system/multi-user.target.wants"
-    if [[ -f "$WPA_TEMPLATE" ]]; then
-        mkdir -p "$WPA_WANTS"
-        ln -sf /lib/systemd/system/wpa_supplicant@.service \
-            "${WPA_WANTS}/wpa_supplicant@wlan0.service"
-    fi
-
     # Disable the generic wpa_supplicant.service — it conflicts with @wlan0 instance
     # Both would fight over wlan0, causing WiFi instability and SSH drops
     rm -f "${MOUNT_POINT}/etc/systemd/system/multi-user.target.wants/wpa_supplicant.service"
@@ -598,6 +589,31 @@ chmod 700 "$SSH_DIR"
 chmod 600 "${SSH_DIR}/authorized_keys"
 info "SSH authorized_keys configured"
 
+# Ensure SSH is allowed through nftables (Armbian default drops input)
+step "Configuring nftables firewall..."
+cat > "${MOUNT_POINT}/etc/nftables.conf" << 'NFTEOF'
+#!/usr/sbin/nft -f
+flush ruleset
+
+table inet filter {
+    chain input {
+        type filter hook input priority 0; policy drop;
+        ct state established,related accept
+        iif lo accept
+        ip protocol icmp accept
+        ip6 nexthdr icmpv6 accept
+        tcp dport 22 accept
+    }
+    chain forward {
+        type filter hook forward priority 0; policy drop;
+    }
+    chain output {
+        type filter hook output priority 0; policy accept;
+    }
+}
+NFTEOF
+info "nftables config written (SSH allowed)"
+
 # Hostname
 step "Setting hostname..."
 echo "$BB_HOSTNAME" > "${MOUNT_POINT}/etc/hostname"