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

docs/email-previews/README.md

@@ -0,0 +1,11 @@
+# Email previews
+
+Static renders of branded HTML emails (dark neon site UX: `#0a0a0a` / `#1a1a1a` / `#00f3ff` / `#bc13fe`).
+
+| Template | Screenshot |
+|----------|------------|
+| Contact notify | [contact.png](contact.png) |
+| Marketing | [marketing.png](marketing.png) |
+| Invoice / pay link | [invoice.png](invoice.png) |
+
+Shared base: `public/templates/emails/base_email.html`.