feat(mailer): redesign emails with Zero Day banner layout

[balebbae]

Summary

• Moves all 8 emails (magic link, decisions, decisions released, QR, walk-in queued/accepted) onto a shared light layout in internal/mailer/template/layout.html: full-bleed Zero Day banner over a red rule, bold heading, label/value detail rows, red CTA button with caption, signature, link band, and address footer.
• Each email file now only defines its title, preheader, heading, and content blocks. Copy and template data are unchanged, so no sender code changed.
• renderTemplate parses the layout alongside each email and registers small helpers (row, accentRow, button, brandImage) so emails can pass several values to shared blocks.
• The banner (internal/mailer/assets/zero-day-banner.jpg, 1200×273, 82 KB) is compiled into the binary with go:embed, replacing the multi-path disk lookup and the Dockerfile copy step. JPEG rather than WebP because Outlook for Windows can't display WebP.

Notes

• Event dates, venue, contact links, and address are hardcoded in layout.html (not settings yet), so they're a one-file edit.
• No unsubscribe link: these are transactional emails.
• The layout declares a light-only color scheme; the Gmail app still inverts by luminance.
• The Vite plugin serving /email-assets/zero-day-title.webp is left in place for previously sent emails that may reference it.

Test plan

• go vet ./..., go test ./internal/mailer/ ./cmd/api/
• Rendered previews via EMAIL_PREVIEW_DIR and checked against the design
• Send a test email through SendGrid/SMTP and check Gmail, Apple Mail, and Outlook

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