Collect Android browser sign-ins after the app was stopped

The Android app kept a pending browser sign-in in a thread inside the
login screen. Android often kills the process while the browser is in
front, so the approval reached the server but nothing collected it, and
the app came back signed out with nothing saved.

The pending device code is now kept in Prefs and collected whenever the
login screen resumes. The app passes return_url=keeporg://device-approved
when it starts the sign-in, and the server's approval page, after a
password or single sign-on, links back to it. Only the keeporg: scheme is
accepted. Against a server older than 0.9, which rejects the field, the
app retries without it.

Verified on an Android 15 emulator: approval collected after the process
was killed, and the fallback against the 0.8 server.

Server 0.9.0, Android 0.3.0 (versionCode 3).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/auth.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"net/http"
+	"net/url"
 	"strings"
 	"time"
 
@@ -187,15 +188,22 @@ func (s *Server) handleTOTPConfirm(w http.ResponseWriter, r *http.Request, u *st
 
 func (s *Server) handleDeviceStart(w http.ResponseWriter, r *http.Request) {
 	var req struct {
-		Label string `json:"label"`
+		Label     string `json:"label"`
+		ReturnURL string `json:"return_url"`
 	}
 	if r.ContentLength > 0 && !readJSON(w, r, &req) {
 		return
 	}
+	if req.ReturnURL != "" && !validReturnURL(req.ReturnURL) {
+		fail(w, http.StatusBadRequest, "bad_request",
+			"return_url must use the "+returnScheme+": scheme")
+		return
+	}
 	d := &store.DeviceAuth{
 		DeviceCode: secret.Token(),
 		UserCode:   secret.UserCode(),
 		Label:      req.Label,
+		ReturnURL:  req.ReturnURL,
 		CreatedAt:  time.Now(),
 		ExpiresAt:  time.Now().Add(deviceTTL),
 	}
@@ -276,11 +284,13 @@ func (s *Server) handleDeviceApprove(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	code := strings.ToUpper(strings.TrimSpace(r.FormValue("code")))
+	var returnURL string
 	err := s.st.Update(func(st *store.State) error {
 		d := st.DeviceByUserCode(code)
 		if d == nil || time.Now().After(d.ExpiresAt) {
 			return errors.New("unknown or expired code")
 		}
+		returnURL = d.ReturnURL
 		u, err := checkPassword(st, r.FormValue("username"), r.FormValue("password"), r.FormValue("totp"))
 		if err != nil {
 			return err
@@ -298,10 +308,20 @@ func (s *Server) handleDeviceApprove(w http.ResponseWriter, r *http.Request) {
 	case err != nil:
 		s.renderDevice(w, code, err.Error())
 	default:
-		s.renderDone(w, "Device approved", "You can close this page and return to the application.")
+		s.renderApproved(w, returnURL)
 	}
 }
 
+// returnScheme is the only scheme a device may ask to be sent back to. It
+// names KeepOrg's own clients, so the approval page cannot be turned into a
+// redirect to anywhere else.
+const returnScheme = "keeporg"
+
+func validReturnURL(raw string) bool {
+	u, err := url.Parse(raw)
+	return err == nil && u.Scheme == returnScheme && u.User == nil
+}
+
 // changePassword replaces a user's password after proving the current one.
 // It is the one place a password changes, so the rules live here: the
 // current password is required even though the caller already holds a