chore(api): log X-Forwarded-For (#10035)

### Purpose

Fix https://github.com/syncthing/syncthing/issues/9336

The `emitLoginAttempt` function now checks for the presence of an
`X-Forwarded-For` header. The IP from this header is only used if the
connecting host is either on loopback or on the same LAN.

In the case of a host pretending to be a proxy, we'd still have both IPs
in the logs, which should make this much less critical from a security
standpoint.

### Testing

1. directly via localhost
2. via proxy an localhost

#### Logs

```
[3JPXJ] 2025/04/11 15:00:40 INFO: Wrong credentials supplied during API authorization from 127.0.0.1
[3JPXJ] 2025/04/11 15:03:04 INFO: Wrong credentials supplied during API authorization from 192.168.178.5 proxied by 127.0.0.1
```

#### Event API

```
  {
    "id": 23,
    "globalID": 23,
    "time": "2025-04-11T15:00:40.578577402+02:00",
    "type": "LoginAttempt",
    "data": {
      "remoteAddress": "127.0.0.1",
      "success": false,
      "username": "sdfsd"
    }
  },
  {
    "id": 24,
    "globalID": 24,
    "time": "2025-04-11T15:03:04.423403976+02:00",
    "type": "LoginAttempt",
    "data": {
      "proxy": "127.0.0.1",
      "remoteAddress": "192.168.178.5",
      "success": false,
      "username": "sdfsd"
    }
  }
```

### Documentation

https://github.com/syncthing/docs/pull/907

---------

Co-authored-by: Jakob Borg <jakob@kastelo.net>

lib/osutil/net.go

@@ -8,6 +8,7 @@ package osutil
 
 import (
 	"net"
+	"strings"
 )
 
 // GetInterfaceAddrs returns the IP networks of all interfaces that are up.
@@ -46,6 +47,17 @@ func GetInterfaceAddrs(includePtP bool) ([]*net.IPNet, error) {
 	return nets, nil
 }
 
+func IPFromString(addr string) net.IP {
+	// strip the port
+	host, _, err := net.SplitHostPort(addr)
+	if err != nil {
+		host = addr
+	}
+	// strip IPv6 zone identifier
+	host, _, _ = strings.Cut(host, "%")
+	return net.ParseIP(host)
+}
+
 func IPFromAddr(addr net.Addr) (net.IP, error) {
 	switch a := addr.(type) {
 	case *net.TCPAddr:

lib/osutil/osutil_test.go

@@ -135,3 +135,35 @@ func TestRenameOrCopy(t *testing.T) {
 		}
 	}
 }
+
+func TestIPFromString(t *testing.T) {
+	t.Parallel()
+
+	cases := []struct {
+		in  string
+		out string
+	}{
+		{"192.168.178.1", "192.168.178.1"},
+		{"192.168.178.1:8384", "192.168.178.1"},
+		{"fe80::20c:29ff:fe9a:46d2", "fe80::20c:29ff:fe9a:46d2"},
+		{"[fe80::20c:29ff:fe9a:46d2]:8384", "fe80::20c:29ff:fe9a:46d2"},
+		{"[fe80::20c:29ff:fe9a:46d2%eno1]:8384", "fe80::20c:29ff:fe9a:46d2"},
+		{"google.com", ""},
+		{"1.1.1.1.1", ""},
+		{"", ""},
+	}
+
+	for _, c := range cases {
+		ip := osutil.IPFromString(c.in)
+		var address string
+		if ip != nil {
+			address = ip.String()
+		} else {
+			address = ""
+		}
+
+		if c.out != address {
+			t.Fatalf("result should be %s != %s", c.out, address)
+		}
+	}
+}