✨ Localize transactional emails

[Logan2234]

Summary

• localize all transactional email templates in French and English
• send the signup page locale explicitly and keep Accept-Language as a legacy fallback
• propagate each recipient's stored locale through every email producer
• keep newsletter and moderation editorial content exactly as authored
• add an FR/EN selector to the admin email preview and test-send gallery
• add localized dates, HTML language metadata, and safer dynamic HTML escaping

Verification

• API email suite: 21/21 tests passed
• API full suite: 1,028/1,029 tests passed; the only timeout passed when rerun alone
• Web full suite: 123/124 tests passed; the only timeout's two cases passed when rerun alone
• Web production build passed
• Shared package build passed
• API production build is currently blocked by the existing Fastify 5.12.1/5.12.3 type mismatch in src/main.ts

Scope

This PR covers emails only. In-app and push notification localization remains a separate follow-up.

Feedback: https://feedback.loomkeep.app/admin/feedback?sort=newest&status=%5B%22open%22%5D&post=post_01m11a5ecdecw9tbwp2686s1rj

[codecov]

Codecov Report

❌ Patch coverage is 42.05128% with 113 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
apps/api/src/mail/mail.service.spec.ts	0.000%	38 Missing ⚠️
apps/api/src/mail/mail.i18n.ts	42.857%	16 Missing and 4 partials ⚠️
apps/api/src/auth/auth.service.spec.ts	0.000%	17 Missing ⚠️
apps/api/src/mail/mail.service.ts	74.603%	12 Missing and 4 partials ⚠️
apps/api/src/admin/admin-emails.controller.spec.ts	0.000%	10 Missing ⚠️
...b/src/routes/app/admin/communications/+page.svelte	0.000%	8 Missing ⚠️
apps/api/src/reports/report.service.spec.ts	0.000%	2 Missing ⚠️
apps/api/src/users/users.controller.spec.ts	0.000%	1 Missing ⚠️
apps/web/src/lib/api/admin.ts	0.000%	1 Missing ⚠️

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