feat(api): add configurable landing page at /

Allow server operators to show new players how to install the game
client when they visit the server address in a browser. The page
content (title and HTML body) is fully configurable via config.json
and can be toggled on/off. Uses Go embed for a self-contained dark-
themed HTML template with zero new dependencies.

config/config.go

@@ -254,6 +254,14 @@ type API struct {
 	Banners     []APISignBanner
 	Messages    []APISignMessage
 	Links       []APISignLink
+	LandingPage LandingPage
+}
+
+// LandingPage holds config for the browser-facing landing page at /.
+type LandingPage struct {
+	Enabled bool   // Toggle the landing page on/off
+	Title   string // Page title (e.g. "My Frontier Server")
+	Content string // Body content — supports raw HTML
 }
 
 type APISignBanner struct {