feat(api): organisation email branding and shared mail layout

- Add email branding columns to organisations table (logo, color, reply-to, sender name, footer)
- Create MailBrandingService for resolving per-org branding with defaults
- Create CrewliMailable abstract base class with branded from/reply-to
- Create shared Blade layout (mail.layouts.crewli) with inline CSS
- Refactor Registration*Mail and InvitationMail to extend CrewliMailable
- Add config/crewli.php for platform-wide defaults (portal_url, app_url, logo)
- Add dev-only /mail-preview/{type} route for browser email previewing
- Update Organisation model, resource, and form requests with branding fields

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Models/Organisation.php

@@ -22,6 +22,11 @@ final class Organisation extends Model
         'slug',
         'billing_status',
         'settings',
+        'email_logo_url',
+        'email_primary_color',
+        'email_reply_to',
+        'email_sender_name',
+        'email_footer_text',
     ];
 
     protected function casts(): array