Stand: SMTP-Test, Admin-Mail-Tab, Notifiable-Fix, Lazy-Quill

- Fix: Notifiable-Trait zum User-Model hinzugefuegt (behebt notify()-500er)
- Installer: SMTP-Verbindungstest mit EsmtpTransport + Ueberspringen-Link
- Admin: Neuer E-Mail-Tab mit SMTP-Konfiguration + Verbindungstest
- Admin: Lazy Quill-Initialisierung (nur sichtbare Locale wird geladen)
- Uebersetzungen: 17 neue Mail-Keys in allen 6 Sprachen

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

vendor/league/flysystem/src/InvalidVisibilityProvided.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace League\Flysystem;
+
+use InvalidArgumentException;
+
+use function var_export;
+
+class InvalidVisibilityProvided extends InvalidArgumentException implements FilesystemException
+{
+    public static function withVisibility(string $visibility, string $expectedMessage): InvalidVisibilityProvided
+    {
+        $provided = var_export($visibility, true);
+        $message = "Invalid visibility provided. Expected {$expectedMessage}, received {$provided}";
+
+        throw new InvalidVisibilityProvided($message);
+    }
+}