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]>

docs/features/account-registration.md

@@ -77,6 +77,40 @@ confirmation token's SHA-256 hash is stored; the token expires after 48h
 | `accepted_at`      | DATETIME         | When accepted; NULL while pending / for group links     |
 | `expires_at`       | DATETIME         | Explicit expiry (end of the chosen day); set on every group link, NULL for personal invites (which expire 14 days after creation) |
 
+## Email and password validation
+
+Both are checked on the server on every signup path, and the browser is given a
+matching but *stricter* job so a bad password is caught before submitting.
+
+**Email** — `type="email"` and `required` in the markup, `is_email()` on the
+server, then `email_exists()` for "an account already exists for this email". A
+personal invite fixes the address and the server always uses the invite's own
+value, so a tampered field is ignored rather than validated.
+
+**Password** — `Auth\PasswordPolicy` is the authority. It deliberately 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:
+
+- shorter than `PasswordPolicy::MIN_LENGTH` (8 — NIST SP 800-63B's floor;
+  composition rules like "must contain a symbol" are deliberately **not** used,
+  as they push people towards predictable substitutions),
+- one of the well-known leaked passwords,
+- built from fewer than four distinct characters (`aaaaaaaa`, `abababab`),
+- containing the user's own display name, email, or the part before the `@`.
+
+The nuance happens in the browser. `register.js` scores the password with
+zxcvbn through WordPress's own `password-strength-meter` script and refuses to
+submit below `PasswordPolicy::MIN_SCORE` (2 of 4 — "medium"; enough to stop a
+guessable password without demanding a passphrase to book a piano lesson). The
+thresholds reach JavaScript via `wp_localize_script()` from the same constants
+the server enforces, so the two cannot drift apart.
+
+The verdict is applied with `setCustomValidity()` on the password field 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 needing to know why. zxcvbn's dictionary loads asynchronously, so
+the gate stays open until it arrives — the server is the check that always runs.
+
 ## Registration Questions (signup step two)
 When the studio has configured **account-scope** registration questions
 (**Offerings → Questions → "Account signup"**, see `registration-questions.md`), the