feat(i18n): add landing payments and not found error pages

[LoneRifle]

Problem and Solution

Closes #8430
Merges #9497

[Copilot]

Pull request overview

This PR internationalizes the frontend’s Not Found and Payments landing pages by extracting previously hardcoded English UI copy into react-i18next locale resources, aligning these pages with the app’s existing i18n structure under translation.features.*.

Changes:

• Replaced hardcoded strings in NotFoundErrorPage with t('features.notFoundError.*') keys.
• Replaced hardcoded strings in LandingPaymentsPage with t('features.landingPayments.*') keys (hero, features, help center/FAQ, bottom CTA, onboarding messages).
• Added new locale feature modules (landing-payments, not-found-error) and wired them into the locale type definitions and en-sg fallback bundle.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
apps/frontend/src/pages/NotFoundError/NotFoundErrorPage.tsx	Swaps static 404 page copy to i18n keys.
apps/frontend/src/pages/Landing/Payments/LandingPaymentsPage.tsx	Swaps payments landing page copy and onboarding messages to i18n keys.
apps/frontend/src/i18n/locales/types.ts	Extends translation typings to include landingPayments and notFoundError.
apps/frontend/src/i18n/locales/features/not-found-error/index.ts	Defines the NotFoundError locale shape and exports en-sg.
apps/frontend/src/i18n/locales/features/not-found-error/en-sg.ts	Adds English (SG) strings for the 404 page.
apps/frontend/src/i18n/locales/features/landing-payments/index.ts	Defines the LandingPayments locale shape and exports en-sg.
apps/frontend/src/i18n/locales/features/landing-payments/en-sg.ts	Adds English (SG) strings for the payments landing page.
apps/frontend/src/i18n/locales/features/index.ts	Exposes new feature locale types from the features barrel.
apps/frontend/src/i18n/locales/en-sg.ts	Registers the new feature locale bundles in the enSG fallback translation.

Comments suppressed due to low confidence (1)

apps/frontend/src/pages/Landing/Payments/LandingPaymentsPage.tsx:110

• handleSubmit closes over t, fieldId, and onboardingSuccessHelperText but they are missing from the useCallback dependency array. This can lead to stale translations after a locale change and will trigger react-hooks/exhaustive-deps warnings.

  const handleSubmit = useCallback(async () => {
    const fieldValue = formMethods.getValues(fieldId) as VerifiableFieldValues
    try {
      await sendOnboardingEmailMutation.mutateAsync({
        email: fieldValue.value,
      })
      formMethods.clearErrors(fieldId)
      setOnboardingHelperText(onboardingSuccessHelperText)
    } catch (error) {
      if (error instanceof Error) {
        if (error.message === 'Forbidden') {
          formMethods.setError(fieldId, {
            message: t('features.landingPayments.onboarding.errors.forbidden'),
          })
        } else {
          formMethods.setError(fieldId, {
            message: t('features.landingPayments.onboarding.errors.general'),
          })
        }
      }
    }
  }, [sendOnboardingEmailMutation, formMethods])