Add one-time setup links as an alternative to the QR code

A config can now be handed over as a one-time link, valid for 1 h, 24 h or
7 days and protected by a PIN by default. Keys are made only when the link
is opened; the link works once and is revoked after 5 wrong PINs. Issuing a
new config offers the same choice, and the current config keeps working
until the link is used.

Remove the option to paste a client's public key, in the web UI, the API
and the iOS app.

web.go

@@ -8,14 +8,14 @@ import (
 // The web UI and its icons are built into the binary. The UI talks only to
 // /api/v1, the same API the iOS app uses.
 //
-//go:embed index.html app.js app.css favicon.svg apple-touch-icon.png
+//go:embed index.html setup.html app.js setup.js app.css favicon.svg apple-touch-icon.png
 var webFiles embed.FS
 
 func webHandler() http.Handler {
 	files := http.FileServerFS(webFiles)
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		switch r.URL.Path {
-		case "/", "/app.js", "/app.css", "/favicon.svg", "/apple-touch-icon.png":
+		case "/", "/app.js", "/setup.js", "/app.css", "/favicon.svg", "/apple-touch-icon.png":
 			w.Header().Set("Cache-Control", "no-cache")
 			files.ServeHTTP(w, r)
 		case "/favicon.ico":
@@ -26,3 +26,16 @@ func webHandler() http.Handler {
 		}
 	})
 }
+
+// setupPage serves the page a setup link opens. The token stays in the URL;
+// setup.js reads it from there and talks to /api/v1/setup.
+func setupPage(w http.ResponseWriter, r *http.Request) {
+	b, err := webFiles.ReadFile("setup.html")
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.Header().Set("Content-Type", "text/html; charset=utf-8")
+	w.Header().Set("Cache-Control", "no-store")
+	_, _ = w.Write(b)
+}