Fix nftables auto-merge syntax and block private/reserved ranges

auto-merge is a standalone set statement, not an nft "flags" value -
"flags interval, auto-merge;" is a syntax error; fixed to "flags
interval;" followed by "auto-merge" on its own line. Needed because large
public blocklists (FireHOL, Spamhaus) contain overlapping CIDRs that
nftables otherwise refuses as "conflicting intervals".

Also add a hard guard: reject any "block" entry that overlaps a private/
reserved/bogon range (RFC1918, CGNAT, loopback, link-local, etc.) in both
the single-entry and bulk-import paths. Public feeds like FireHOL level1
routinely include ranges like 10.0.0.0/8 and 172.16.0.0/12, meant for
WAN-only edge firewalls - applied host-wide here (where WireGuard/LAN
subnets legitimately live in that same private space), those entries
would silently block a server's own internal/VPN traffic instead of
actual bad actors.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

handler/routes.go

@@ -1789,6 +1789,53 @@ func FirewallListsPage() echo.HandlerFunc {
 	}
 }
 
+// dangerousBlockRanges are private/reserved/bogon IPv4 supernets that public
+// threat-intel feeds (FireHOL level1, some Spamhaus-derived lists, etc.)
+// often include, since they're meaningless on a public WAN edge. On a host
+// that also uses private address space internally (LAN, WireGuard client
+// subnets - as this one does), blocking these at the global (host-wide)
+// priority-10 hook would drop that host's own internal/VPN traffic. So
+// "block" entries overlapping any of these are rejected outright; "allow"
+// entries are never affected.
+var dangerousBlockRanges = mustParseCIDRs([]string{
+	"0.0.0.0/8", "10.0.0.0/8", "100.64.0.0/10", "127.0.0.0/8",
+	"169.254.0.0/16", "172.16.0.0/12", "192.168.0.0/16",
+	"192.0.0.0/24", "192.0.2.0/24", "198.18.0.0/15", "198.51.100.0/24",
+	"203.0.113.0/24", "224.0.0.0/4", "240.0.0.0/4",
+})
+
+func mustParseCIDRs(cidrs []string) []*net.IPNet {
+	nets := make([]*net.IPNet, 0, len(cidrs))
+	for _, c := range cidrs {
+		_, n, err := net.ParseCIDR(c)
+		if err != nil {
+			panic(err)
+		}
+		nets = append(nets, n)
+	}
+	return nets
+}
+
+// overlapsDangerousRange reports whether cidr (an IP or CIDR string)
+// overlaps any private/reserved/bogon supernet in dangerousBlockRanges.
+func overlapsDangerousRange(cidr string) bool {
+	var ipNet *net.IPNet
+	if ip, n, err := net.ParseCIDR(cidr); err == nil {
+		ipNet = n
+		_ = ip
+	} else if ip := net.ParseIP(cidr); ip != nil {
+		ipNet = &net.IPNet{IP: ip, Mask: net.CIDRMask(32, 32)}
+	} else {
+		return false
+	}
+	for _, dangerous := range dangerousBlockRanges {
+		if dangerous.Contains(ipNet.IP) || ipNet.Contains(dangerous.IP) {
+			return true
+		}
+	}
+	return false
+}
+
 func validateIPListEntry(entry model.IPListEntry) error {
 	if entry.ListType != "allow" && entry.ListType != "block" {
 		return fmt.Errorf("list_type must be 'allow' or 'block'")
@@ -1796,6 +1843,9 @@ func validateIPListEntry(entry model.IPListEntry) error {
 	if !util.ValidateServerAddresses([]string{entry.CIDR}) && net.ParseIP(entry.CIDR) == nil {
 		return fmt.Errorf("cidr must be a valid IP or CIDR")
 	}
+	if entry.ListType == "block" && overlapsDangerousRange(entry.CIDR) {
+		return fmt.Errorf("refusing to block %s: overlaps a private/reserved range - this host uses private address space internally (LAN/WireGuard subnets), blocking it here would cut off your own traffic", entry.CIDR)
+	}
 	if len(entry.Comment) > 200 {
 		return fmt.Errorf("comment too long")
 	}