New handler tests in internal/handlers, with no network: GET / shows the
login form without a session; a wrong key shows it again with an error and
sets no session cookie; the right key answers 303 with a session cookie
marked Secure, HttpOnly and SameSite=Strict, with which GET / shows the
generator page; GET /logout empties the cookie with Max-Age=0; POST
/generate without a session answers 303 to /; /v1/e/ serves a valid
token's image, answers 410 for an expired token and 400 for one changed,
cut short or made with another signing key; a URL made on the generator
page is served by /v1/e/. No code changes.
Model: opus-5-5
