Fix single sign-on from the web, and make passwords changeable

Why: two questions about operating this turned up two gaps. "Where do I get
the admin password" had no answer once the line scrolled out of the log,
and "how do I set up OIDC" ran straight into a sign-in link that could not
work.

What:
- /oidc/login with no parameters now signs a browser in and ends in a
  session cookie. It previously required the user code of a waiting client,
  so the link on the web sign-in page always failed. With a code it still
  approves a device, and the two paths no longer overlap: an empty user
  code matches no pending device.
- POST /api/v1/auth/password, `keeporg passwd`, and a form on /web/vaults.
  Changing a password requires the current one, and the one-time code when
  a second factor is enrolled, and revokes that account's other sessions.
  Until now nothing could change a password, which made the advice printed
  at first start impossible to follow.
- keeporgd -reset-admin, for when that first password was not kept: a new
  one printed once, any second factor cleared, sessions revoked. It writes
  the state document directly and says to stop the server first.
- The client read stdin through a new buffered reader per prompt, so the
  first prompt swallowed the rest and anything asking twice could not be
  scripted. One shared reader now. A prompt for a password being set no
  longer reads $KEEPORG_PASSWORD either; that variable holds the password
  you sign in with, and taking it here would have set the new password to
  the old one.
- HANDOVER gains a first-sign-in section and an OIDC setup section covering
  the redirect URI, the config block, both entry points and how an identity
  becomes an account.

Proved: OIDC now has tests, against a stub provider that serves discovery,
authorization, token and userinfo. They cover the web sign-in that was
broken, device approval, linking an existing account by username, the
refusal when auto_create is off, and a forged callback state. Eight more
cover the password change. Checked by hand with the real binaries: the
first-start password logs once and works, -reset-admin replaces it while
the old one stops working, and `keeporg passwd` changes it from a pipe.
Server 0.5.0, desktop clients 0.3.0.

internal/server/web.go

@@ -176,6 +176,45 @@ func (s *Server) handleWebCreateVault(w http.ResponseWriter, r *http.Request) {
 			"with the password you just set.")
 }
 
+func (s *Server) handleWebChangePassword(w http.ResponseWriter, r *http.Request) {
+	sess := s.webUser(r)
+	if sess == nil {
+		s.renderSignIn(w, "")
+		return
+	}
+	if err := r.ParseForm(); err != nil || !s.checkCSRF(r, sess) {
+		fail(w, http.StatusForbidden, "csrf", "stale form, reload the page")
+		return
+	}
+	next := r.FormValue("new_password")
+	if next != r.FormValue("new_password2") {
+		s.renderVaults(w, sess, "The two new passwords do not match.", "")
+		return
+	}
+	cookie, _ := r.Cookie(webCookie)
+	keep := ""
+	if cookie != nil {
+		keep = cookie.Value
+	}
+	err := s.st.Update(func(st *store.State) error {
+		return changePassword(st, sess.user.Username, r.FormValue("current_password"),
+			r.FormValue("totp"), next, keep)
+	})
+	switch {
+	case errors.Is(err, errTOTPRequired):
+		s.renderVaults(w, sess, "Enter your one-time code to change the password.", "")
+	case errors.Is(err, errTOTPInvalid):
+		s.renderVaults(w, sess, "That one-time code was rejected.", "")
+	case errors.Is(err, errBadCredentials):
+		s.renderVaults(w, sess, "The current password is wrong.", "")
+	case err != nil:
+		s.renderVaults(w, sess, err.Error(), "")
+	default:
+		s.renderVaults(w, sess, "",
+			"Password changed. Other sessions on this account were signed out.")
+	}
+}
+
 // humanError keeps internal wording out of the browser.
 func humanError(err error) string {
 	if errors.Is(err, errForbidden) {