feat: packages/email — Maizzle v6 + Tailwind CSS v4 HTML email templates

- New @sexy.pivoine.art/email package with @maizzle/framework@6.0.0-15
- Uses @maizzle/tailwindcss (TW v4 preset) with @theme brand tokens
  derived from the frontend's app.css oklch primary color
- LightningCSS automatically lowers oklch/lab to hex for email clients
- Real HTML template files (templates/layouts/main.html, verification.html,
  password-reset.html) — not JS template strings
- PostCSS `from` override so @import "@maizzle/tailwindcss" resolves from
  the email package's own node_modules
- Backend lib/email.ts now calls renderVerification/renderPasswordReset
  instead of inline HTML strings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/backend/package.json

@@ -20,6 +20,7 @@
     "@fastify/static": "^8.1.1",
     "@pothos/core": "^4.4.0",
     "@pothos/plugin-errors": "^4.2.0",
+    "@sexy.pivoine.art/email": "workspace:*",
     "@sexy.pivoine.art/types": "workspace:*",
     "argon2": "^0.43.0",
     "bullmq": "^5.70.4",