Settings gear in topbar linking to placeholder /settings page (#36)

- layout.html: inline SVG gear icon top-right, styled as .iconbtn.gear pill
- new settings.html template reusing standard layout with under-construction card
- register GET /settings route in main.go

web.go

@@ -66,6 +66,10 @@ func (a *apiServer) handleHistoryPage(w http.ResponseWriter, r *http.Request) {
 	renderPage(w, "history.html", map[string]any{"Page": "history"})
 }
 
+func (a *apiServer) handleSettingsPage(w http.ResponseWriter, r *http.Request) {
+	renderPage(w, "settings.html", map[string]any{"Page": "settings"})
+}
+
 func agoString(ts int64) string {
 	s := time.Now().Unix() - ts
 	switch {