feat(forms): add respondent confirmation emails with optional email verification

[QuiteBitter]

What this changes

• Adds respondent confirmation mails for short questions that use email validation and are explicitly marked as the confirmation recipient in extraSettings.
• Adds an optional email verification flow for that recipient question. When verification is enabled, submissions stay unverified until the recipient uses the mailed verification link.
• Sends the confirmation mail only after the submission is verified, so verified flows do not acknowledge unverified addresses.
• Adds the public verification route, token persistence, and unit/integration coverage for the full submit -> verify -> confirm path.

Related issues

• email validation for anonymous respondents #1418
• Confirmation mail for respondents #525

Upstream context

• feat: Add confirmation email for form respondents #3098 already proposes confirmation mails. This branch intentionally follows the review direction there by using an explicit recipient flag on the question instead of auto-picking an email field, and then extends that flow with email verification.
• forms currently supports Nextcloud 31-33. OCP\\Mail\\IEmailValidator is only available from Nextcloud 32 onward, so this implementation keeps IMailer::validateMailAddress() for 31 compatibility.

Scope / non-goals

• Uses the configured system mail account and a fixed branded template.
• Does not add per-form sender selection or custom subject/body templating yet.
• Leaves generated locale catalogs out of the PR because Nextcloud translations are handled through Transifex.

Validation

• Nextcloud 33: unit and integration suites passed.
• Nextcloud 31: unit and integration suites passed.

[Chartman123]

Thanks for your PR! Please understand that we're currently migrating to Vue3 and won't merge new features until this has finished to minimize extra work.

Other than that: #525 is already addressed in #3098

[QuiteBitter]

Thanks for your PR! Please understand that we're currently migrating to Vue3 and won't merge new features until this has finished to minimize extra work.

Other than that: #525 is already addressed in #3098

Okey, I will wait for this to complete :) good luck! After completion, I'll check what's needed to be done on my side.