Validate signup email and password strength

The password was only ever checked for length. It is now checked on both
sides, with each side doing the job it can actually do.

The browser scores it with zxcvbn, through WordPress's own
password-strength-meter script rather than a second opinion of our own, and
refuses to submit below "medium". That is the nuanced test — it knows
Tr0ub4dor&3 is weaker than it looks — but it is advice a client can decline
to take.

Auth\PasswordPolicy runs on the server and is the rule that holds. It does
not try to reproduce a strength score in PHP; it rejects the categorically
bad, which is what a server can check without shipping a dictionary: too
short, a well-known leaked password, fewer than four distinct characters, or
the user's own name or email inside it. No composition rules — NIST advises
against them, and they mostly produce predictable substitutions.

Both thresholds come from the same two constants, handed to JavaScript by
wp_localize_script, so the sides cannot drift into disagreeing about what
was accepted.

The verdict is attached to the field with setCustomValidity() rather than by
disabling a button. The form has up to three submits plus a "Next" that
already gates on checkValidity(), and an invalid field stops all of them
without any of them needing to know why.

Email validation moved ahead of the password check, since the password is
now checked against the email. A blank form therefore reports the email
first, which also matches the order the fields appear in.

Verified the browser half against a controllable scorer: each score band
blocks or allows as intended, the identity list reaches the meter, and the
gate stays open while zxcvbn's dictionary is still loading — the server
covers that window.

Closes #150

Co-Authored-By: Claude Opus 5 <[email protected]>

templates/frontend/register-page.php

@@ -1,6 +1,7 @@
 <?php
 declare(strict_types=1);
 
+use Unsupervised\Schedular\Auth\PasswordPolicy;
 use Unsupervised\Schedular\Registration\Question;
 use Unsupervised\Schedular\Registration\QuestionField;
 
@@ -67,7 +68,15 @@ if (! defined('ABSPATH')) {
                     </p>
                     <p>
                         <label for="us-reg-pass"><?php esc_html_e('Password', 'unsupervised-schedular'); ?></label>
-                        <input type="password" name="password" id="us-reg-pass" autocomplete="new-password" minlength="8" required>
+                        <input type="password" name="password" id="us-reg-pass" autocomplete="new-password" minlength="<?php echo esc_attr((string) PasswordPolicy::MIN_LENGTH); ?>" required aria-describedby="us-reg-pass-strength">
+                        <?php
+                        /*
+                         * Filled in by register.js. `aria-live` announces the verdict as
+                         * it changes, and it starts empty so nothing is announced — or
+                         * takes up space — before anything has been typed.
+                         */
+                        ?>
+                        <span class="us-password-strength" id="us-reg-pass-strength" role="status" aria-live="polite"></span>
                     </p>
 
                     <fieldset class="us-guardian">