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/Mail/CrewliMailable.php

@@ -0,0 +1,41 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Mail;
+
+use App\Models\Organisation;
+use App\Services\MailBrandingService;
+use Illuminate\Bus\Queueable;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Mail\Mailable;
+use Illuminate\Queue\SerializesModels;
+
+abstract class CrewliMailable extends Mailable implements ShouldQueue
+{
+    use Queueable;
+    use SerializesModels;
+
+    protected Organisation $organisation;
+
+    public function __construct(Organisation $organisation)
+    {
+        $this->organisation = $organisation;
+    }
+
+    protected function buildBranding(): static
+    {
+        $branding = app(MailBrandingService::class)->getBranding($this->organisation);
+
+        $this->from(
+            config('mail.from.address'),
+            $branding['sender_name']
+        );
+
+        if ($branding['reply_to']) {
+            $this->replyTo($branding['reply_to']);
+        }
+
+        return $this->with('branding', $branding);
+    }
+}