Align emails with site UX (#26) (#27)

## Summary
- Closes #26
- Shared dark branded email base (`emails/base_email.html`) matching site tokens: `#0a0a0a` / `#1a1a1a`, cyan `#00f3ff`, purple `#bc13fe`, logo, Inter-safe stack
- Refactors contact, marketing (+ preview), and invoice pay-link HTML/text to extend that base; drops Materialize CDN
- Adds `PUBLIC_SITE_URL` + `email_brand_context()` for absolute logo/footer links

## Email renders

### Contact
![Contact email](https://git.aimloperations.com/ai_ml_operations/company_site/raw/branch/feature/26-brand-email-ux/docs/email-previews/contact.png)

### Marketing
![Marketing email](https://git.aimloperations.com/ai_ml_operations/company_site/raw/branch/feature/26-brand-email-ux/docs/email-previews/marketing.png)

### Invoice pay link
![Invoice email](https://git.aimloperations.com/ai_ml_operations/company_site/raw/branch/feature/26-brand-email-ux/docs/email-previews/invoice.png)

Also see `docs/email-previews/`.

## Test plan
- [ ] Preview a marketing `EmailMessage` in admin / `/preview_email/<pk>/` — dark brand, logo, no Materialize
- [ ] Submit contact form (or render `emails/contact_email.html`) — subject/from/message fields + branded footer
- [ ] Send (or dry-render) invoice pay-link email — cyan CTA, amount accent
- [ ] Confirm logo loads from `PUBLIC_SITE_URL/static/public/img/logo.png` in a real clientReviewed-on: #27

.env.example

@@ -34,6 +34,9 @@ EMAIL_HOST_PASSWORD=
 EMAIL_PORT=2525
 EMAIL_USE_TLS=true
 # DEFAULT_FROM_EMAIL=AI ML Operations, LLC <info@aimloperations.com>
+# Absolute origin for email logo / footer links
+PUBLIC_SITE_URL=https://aimloperations.com
+# DEFAULT_FROM_EMAIL=AI ML Operations, LLC <info@aimloperations.com>
 
 # Stripe (test keys from https://dashboard.stripe.com/test/apikeys)
 STRIPE_SECRET_KEY=