Fix UTF-8 BOM in dnsmasq config/hosts writes so dnsmasq does not fail

- Add DnsmasqFileEncoding.Utf8NoBom for all config and hosts file writes
- Strip BOM from first line when reading main config and managed config
- Prevents 'bad option at line 1' when dnsmasq loads files written by the app

src/DnsmasqWebUI/Infrastructure/DnsmasqFileEncoding.cs

@@ -0,0 +1,9 @@
+using System.Text;
+
+namespace DnsmasqWebUI.Infrastructure;
+
+/// <summary>Encoding for dnsmasq config and hosts files. UTF-8 without BOM so dnsmasq does not see a BOM as "bad option" on line 1.</summary>
+public static class DnsmasqFileEncoding
+{
+    public static readonly Encoding Utf8NoBom = new UTF8Encoding(false);
+}