Email subscription for site updates and new tool announcements

[YurMil]

Add an opt-in email subscription (footer block + occasional inline prompt on docs pages): store subscriptions in Supabase with double opt-in and an unsubscribe link, and a simple way to compose/send an update to subscribers.

Why: the site currently has no way to tell users about new utilities or improvements; visitors who liked a tool have no channel to come back through.

Acceptance criteria:

• Subscription form (localized), double opt-in, one-click unsubscribe
• RLS-protected storage; email used for nothing else (privacy note)
• Free-tier friendly sending path (e.g. Supabase Edge Function + free email API tier)
• GDPR-conscious: explicit consent text, data deletion honored by account deletion flow

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