A guest login: temporary, look-only, revocable

Users -> Guest logins makes a throwaway user name and password, good for half
an hour by default, to hand to somebody who wants to see the console. It is a
normal account with `guest: true` and an expiry, so login, sessions and the
saved look need no special case, and it is left out of the account list, out of
"the last account can't be removed", and out of the one account an empty user
name means.

What a guest may do is one allow-list, checked in the auth middleware, rather
than a flag ~190 handlers have to remember. The list is matched by asking the
router which handler a request would reach, so it cannot drift from the routes
it names, and it is checked against the scoped path — /api/nodes/grid/procs is
/api/procs on grid — because the hub proxies to a node with a token that is
full control of that node. The node cannot tell a guest from its owner, so the
only place it can be stopped is here, before the request leaves.

A guest gets the dashboard, containers, stacks, disks, hardware, ZFS and usage,
on this machine and on any managed one, and nothing else: no terminal (attaching
is a GET, and it is a root shell), no files, no container logs or environments,
no stack files, no alert channels, no cron, no tokens, no settings, no power.

Expiry is enforced on every request and swept once a minute, so a console left
open on a visitor's phone stops within the minute rather than at their next
click; revoking signs them out at once. The console hides what a guest cannot
use and says how long is left where the power button would be.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/auth/auth.go

@@ -113,11 +113,14 @@ var dummyHash = func() string {
 	return h
 }()
 
-// CheckUser verifies a password for one of users. A missing user fails in the same time
-// as a wrong password.
+// CheckUser verifies a password for one of users. A missing user — or a guest login whose
+// time is up — fails in the same time as a wrong password.
 func CheckUser(users []config.User, name, pw string) bool {
 	for _, u := range users {
 		if u.Name == name {
+			if u.Expired(time.Now()) {
+				break
+			}
 			return VerifyPassword(u.Hash, pw)
 		}
 	}
@@ -125,6 +128,51 @@ func CheckUser(users []config.User, name, pw string) bool {
 	return false
 }
 
+// ---- guest logins ----
+
+// guestAlphabet leaves out every letter and digit that can be misread in a screenshot or
+// over the phone: i, l, o, 0 and 1. A guest login is read aloud and typed by hand.
+const guestAlphabet = "abcdefghjkmnpqrstuvwxyz23456789"
+
+// randomFrom returns n characters of guestAlphabet, chosen without modulo bias.
+func randomFrom(n int) (string, error) {
+	out := make([]byte, 0, n)
+	buf := make([]byte, n)
+	limit := byte(256 - 256%len(guestAlphabet)) // the top of the last whole run of the alphabet
+	for len(out) < n {
+		if _, err := rand.Read(buf); err != nil {
+			return "", err
+		}
+		for _, b := range buf {
+			if b >= limit {
+				continue // would favour the first few letters; ask for another byte instead
+			}
+			out = append(out, guestAlphabet[int(b)%len(guestAlphabet)])
+			if len(out) == n {
+				break
+			}
+		}
+	}
+	return string(out), nil
+}
+
+// GuestPassword is a password for a guest login: four groups of four, ~78 bits, in an
+// alphabet with nothing ambiguous in it.
+func GuestPassword() (string, error) {
+	var groups []string
+	for i := 0; i < 4; i++ {
+		g, err := randomFrom(4)
+		if err != nil {
+			return "", err
+		}
+		groups = append(groups, g)
+	}
+	return strings.Join(groups, "-"), nil
+}
+
+// GuestSuffix is the tail of a generated guest name.
+func GuestSuffix() (string, error) { return randomFrom(4) }
+
 // ---- sessions ----
 
 // Sessions is a token store backed by the state file. Expiry is sliding.