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.

server/api/landing_page.go

@@ -0,0 +1,35 @@
+package api
+
+import (
+	_ "embed"
+	"html/template"
+	"net/http"
+)
+
+//go:embed landing_page.html
+var landingPageHTML string
+
+var landingPageTmpl = template.Must(template.New("landing").Parse(landingPageHTML))
+
+type landingPageData struct {
+	Title   string
+	Content template.HTML
+}
+
+// LandingPage serves a configurable HTML landing page at /.
+func (s *APIServer) LandingPage(w http.ResponseWriter, r *http.Request) {
+	lp := s.erupeConfig.API.LandingPage
+	if !lp.Enabled {
+		http.NotFound(w, r)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	data := landingPageData{
+		Title:   lp.Title,
+		Content: template.HTML(lp.Content),
+	}
+	if err := landingPageTmpl.Execute(w, data); err != nil {
+		s.logger.Error("Failed to render landing page")
+		http.Error(w, "Internal Server Error", http.StatusInternalServerError)
+	}
+}