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/Http/Resources/Api/V1/OrganisationResource.php

@@ -17,6 +17,11 @@ final class OrganisationResource extends JsonResource
             'slug' => $this->slug,
             'billing_status' => $this->billing_status,
             'settings' => $this->settings,
+            'email_logo_url' => $this->email_logo_url,
+            'email_primary_color' => $this->email_primary_color,
+            'email_reply_to' => $this->email_reply_to,
+            'email_sender_name' => $this->email_sender_name,
+            'email_footer_text' => $this->email_footer_text,
             'created_at' => $this->created_at->toIso8601String(),
             'updated_at' => $this->updated_at->toIso8601String(),
         ];