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

main.go

@@ -367,6 +367,7 @@ func (a *apiServer) routes() http.Handler {
 	r.Get("/", http.RedirectHandler("/history", http.StatusFound).ServeHTTP)
 	r.Get("/new", a.handleNewPage)
 	r.Get("/history", a.handleHistoryPage)
+	r.Get("/settings", a.handleSettingsPage)
 	r.Handle("/static/*", staticHandler())
 	r.Get("/unlock/{id}", a.handlePasteView)
 	r.Post("/unlock/{id}", a.handlePasteView)