fix(user): Add user email as part of registration in authentication

[Mrudhulraj]

What changes were proposed in this PR?

This PR fixes a bug in local-account registration where the user's email was
not captured or stored. Two layers were silently dropping the email:

Layer	Before	After
Frontend Sign Up form	No email input	New required email input with format validation
auth.service.ts → POST /auth/register	Sent { username, password }	Sends { username, email, password }
UserRegistrationRequest (Scala case class)	(username, password)	(username, email, password)
AuthResource.register	user.setEmail(username)	user.setEmail(request.email)

Before, every locally-registered user had email = username in the database,
because the request DTO had no email field and the resource fell back to
setEmail(username). After this PR, the user-entered email is what lands in
the user.email column.

Frontend-side, the new registerEmail form control uses
[Validators.required, Validators.email]. A new UserService.validateEmail
does a defensive non-empty + regex check before the request is sent. A new
#emailErrorTip template renders "Please input your email." / "Please input
a valid email." messages on the Sign Up tab.

Any related issues, documentation, discussions?

How was this PR tested?

Unit tests (frontend, local-login.component.spec.ts):

• builds allForms with the expected controls — asserts the form group now
  includes registerEmail alongside the existing controls.
• requires registerEmail and enforces email format — covers the
  Validators.required and Validators.email behavior on registerEmail.
• register › sets registerErrorMessage when the email is empty — short
  circuits before hitting userService.register when validateEmail fails
  on empty input.
• register › sets registerErrorMessage when the email is malformed —
  covers an invalid email format path.
• register › checks email validity before username validity — confirms the
  validation order: email runs before username when both are bad.
• register › calls UserService.register with the trimmed username and surfaces a success notification — verifies the email value is forwarded
  through userService.register(..., email, ...).
• register › surfaces the error's message via NotificationService.error on failure and register › falls back to 'Registration failed' when the error has no message — error-path coverage on the new flow.

Run command (copy/paste-able):

cd frontend
yarn test --watch=false --testPathPattern=local-login

Backend was verified manually end-to-end: Sign Up tab → submit → row in
user table has the user-entered email rather than the username.

Was this PR authored or co-authored using generative AI tooling?

Yes for frontend forms and testing. Backend was fixed manually.

[github-actions]

👋 Thanks for opening this pull request, @Mrudhulraj!

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[github-actions]

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