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/database/migrations/2026_04_13_100000_add_email_branding_to_organisations_table.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    public function up(): void
+    {
+        Schema::table('organisations', function (Blueprint $table) {
+            $table->string('email_logo_url', 500)->nullable()->after('settings');
+            $table->string('email_primary_color', 7)->nullable()->after('email_logo_url');
+            $table->string('email_reply_to')->nullable()->after('email_primary_color');
+            $table->string('email_sender_name', 100)->nullable()->after('email_reply_to');
+            $table->text('email_footer_text')->nullable()->after('email_sender_name');
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::table('organisations', function (Blueprint $table) {
+            $table->dropColumn([
+                'email_logo_url',
+                'email_primary_color',
+                'email_reply_to',
+                'email_sender_name',
+                'email_footer_text',
+            ]);
+        });
+    }
+};