Sign in as a user instead of with an API key

The web UI signs in with a username and password and holds a session
cookie; the TUI was the only client still needing an API key pasted into
a config file. It now asks for the same credentials on a form at start.

What is kept between runs is the session token, not the password, in
session.json under the config directory, mode 0600 and keyed by server
URL so one server's token is never offered to another. It resumes on the
next start; the server's sessions last 30 days and slide with use. L
signs out, which ends the session on the server and deletes the saved
one even if the server cannot be reached.

The client attaches the cookie by hand instead of using a cookie jar:
the server marks it Secure behind https, and a jar drops a Secure cookie
it is given over plain http, which would break a local server for no
reason. It sends no Authorization header at all, since the server judges
a request carrying one on that alone and never falls back to the cookie.
Writes go through the server's cross-origin guard, which lets a client
that sends neither Origin nor Sec-Fetch-Site through; checked against a
real v0.20.1 server for both reads and writes.

A 401 from anything means the session is gone (expired, ended from the
web UI, or the account disabled), so the TUI returns to the form with the
reason, forgets the saved token, and drops what the last session loaded
rather than showing it to whoever signs in next. A 403 is a permission
and leaves the session alone. The refresh timer is started once, so
signing out and in does not leave two running.

An account with no password cannot sign in, and the server answers it
exactly like a wrong password, so the form's message says a password
must be set first. Users created only for API access hit this.

Breaking: api_key in config.yaml is no longer used. It is not an error
to leave it there; the form says it is ignored. API keys still exist on
the server and k in Users still manages them.

internal/session/session.go

@@ -0,0 +1,98 @@
+// Package session keeps the signed-in session between runs, so the TUI does not
+// ask for a password every time it starts.
+//
+// What is stored is the server's session token, not the password: it is what the
+// web UI keeps in a cookie, it expires on the server's schedule (30 days, sliding
+// with use) and signing out or a password change ends it. It is still a
+// credential, so the file is readable by its owner only.
+package session
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+type file struct {
+	ServerURL string `json:"server_url"`
+	Token     string `json:"token"`
+}
+
+func path() (string, error) {
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, "terdut-tui", "session.json"), nil
+}
+
+// normalise makes two spellings of one server compare equal.
+func normalise(serverURL string) string {
+	return strings.TrimRight(serverURL, "/")
+}
+
+// Load returns the saved token for serverURL, or "" when there is none. A session
+// saved for a different server is not offered to this one, and an unreadable file
+// is the same as no file: the worst outcome is being asked to sign in.
+func Load(serverURL string) string {
+	p, err := path()
+	if err != nil {
+		return ""
+	}
+	data, err := os.ReadFile(p)
+	if err != nil {
+		return ""
+	}
+	var f file
+	if json.Unmarshal(data, &f) != nil || normalise(f.ServerURL) != normalise(serverURL) {
+		return ""
+	}
+	return f.Token
+}
+
+// Save stores the token for serverURL, replacing whatever was there.
+func Save(serverURL, token string) error {
+	p, err := path()
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(p), 0o700); err != nil {
+		return err
+	}
+	data, err := json.Marshal(file{ServerURL: normalise(serverURL), Token: token})
+	if err != nil {
+		return err
+	}
+	// Written beside the target and renamed over it, so a crash cannot leave a
+	// half-written token, and created 0600 so it is never briefly world-readable.
+	tmp, err := os.CreateTemp(filepath.Dir(p), ".session-*")
+	if err != nil {
+		return err
+	}
+	defer os.Remove(tmp.Name())
+	if err := os.Chmod(tmp.Name(), 0o600); err != nil {
+		tmp.Close()
+		return err
+	}
+	if _, err := tmp.Write(data); err != nil {
+		tmp.Close()
+		return err
+	}
+	if err := tmp.Close(); err != nil {
+		return err
+	}
+	return os.Rename(tmp.Name(), p)
+}
+
+// Clear forgets the saved session. Having none to forget is not an error.
+func Clear() error {
+	p, err := path()
+	if err != nil {
+		return err
+	}
+	if err := os.Remove(p); err != nil && !os.IsNotExist(err) {
+		return err
+	}
+	return nil
+}