S1 #7 steps 1-2: IPv4 checks, the IP setting per Saved Network, DNS and NTP settings

The plan and decisions Q105 to Q116 (docs/milestones/S1.md). lib/net:
strict IPv4 parsing, prefix and mask, and the checks a Fixed setting must
pass, each refusal with its reason. A Saved Network is Automatic or Fixed
(address/prefix and an optional gateway), kept with it in flash. Settings:
two DNS servers (9.9.9.9, 1.1.1.1), "Always use my DNS", two NTP servers
(pool.ntp.org, time.cloudflare.com). Host-tested: 383 tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

lib/services/src/settings.h

@@ -24,6 +24,11 @@ enum class Setting : uint8_t {
     GnssEnabled,      // bool: the GNSS Service reads the receiver (M2, Q58)
     CoordinatesDms,   // bool: show degrees, minutes and seconds instead of decimal degrees (Q64)
     LoraPreset,       // int: the LoRa Scanner's Meshtastic preset, an index into the EU868 list (M3, Q95)
+    Dns1,             // string: the first DNS server, an IPv4 address (S1, Q108, Q109)
+    Dns2,             // string: the second, or empty
+    DnsAlways,        // bool: use them on Automatic (DHCP) networks too, instead of DHCP's
+    Ntp1,             // string: the first NTP server, a host name or an IPv4 address (S1, Q110)
+    Ntp2,             // string: the second, or empty
     Count
 };