Show the real wait time after too many login attempts #10

After too many login attempts, the plugin says "Please try again later" without a time. A fixed time in the message would drift from userli's roundcube_login limiter config, and the sliding window makes the real wait shorter anyway.

Proposal

• userli: send a Retry-After header with the 429 response of /api/roundcube/login, from RateLimit::getRetryAfter().
• Plugin: read Retry-After in request_api(), round up to minutes, and pass $this->gettext(['name' => 'too_many_attempts', 'vars' => ['minutes' => $m]]) as $args['error']. Roundcube shows non-label text unchanged. Keep "try again later" when the header is missing (older userli).

Related: userli PR 1394, roundcube-userli-plugin PR 9.

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