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/config/crewli.php

@@ -0,0 +1,7 @@
+<?php
+
+return [
+    'default_logo_url' => env('CREWLI_DEFAULT_LOGO_URL'),
+    'portal_url' => env('CREWLI_PORTAL_URL', 'http://localhost:5175'),
+    'app_url' => env('CREWLI_APP_URL', 'http://localhost:5174'),
+];