Merge pull request 'Validate signup email and password strength' (#155) from feature/150-signup-credential-validation into main

Reviewed-on: #155

CHANGELOG.md

@@ -13,6 +13,9 @@ each change under the current top section as you work.
 
 ## [1.3.1]
 
+### Security
+- Signup now checks the password properly. The form scores it as you type with the same zxcvbn meter wp-admin uses and will not submit a weak one, and the server refuses — regardless of what the browser allowed — anything shorter than 8 characters, one of the well-known leaked passwords, one built from barely any distinct characters, or one containing your own name or email address. Composition rules ("must contain a symbol") are deliberately not imposed: they mostly produce predictable substitutions. Email addresses are validated on the server on every signup path, with a clear message when one is already registered.
+
 ### Changed
 - A student's **name and birth year are now required**, marked in the form the same way a required registration question is and enforced on the server whichever way they were submitted. On signup the requirement applies only once the parent/guardian box is ticked, so registering for yourself is unaffected. A student block you have started filling in is now reported back to you rather than silently dropped when the name is missing — only a completely untouched spare block is still ignored.
 - Signup and the profile page now ask for a **birth year** rather than a full date of birth — a four-digit year between 1900 and the current year, with anything else discarded rather than stored. Students added before this change keep showing a birth year, derived from the date already on file; that old full date is then dropped the first time the record is saved, so the studio ends up holding only what it now asks for. No bulk purge runs, so a site wanting the remaining old dates gone should clear the `us_date_of_birth` user meta directly.