feat(layout): Layout-Einstellungen, Dark-Mode, Logo, Hilfe-Menü

- Settings Key-Value Store (DB + Cache)
- Einstellungen → Layout: Seitenname, Logo, Button-Farbe, Dark/Light-Mode
- Hilfe-Menü (Ebene 0): Handbuch + Changelog im Browser
- Navigation erweitert: Einstellungen-Dropdown + Hilfe-Dropdown
- CHANGELOG v0.4.0

Version: 0.4.0

app/Services/SettingsService.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace App\Services;
+
+use App\Models\Setting;
+use Illuminate\Support\Facades\Cache;
+
+class SettingsService
+{
+    private const CACHE_KEY = 'app_settings';
+    private const CACHE_TTL = 3600;
+
+    public function all(): array
+    {
+        return Cache::remember(self::CACHE_KEY, self::CACHE_TTL, fn() => Setting::allAsArray());
+    }
+
+    public function get(string $key, mixed $default = null): mixed
+    {
+        return $this->all()[$key] ?? $default;
+    }
+
+    public function set(string $key, mixed $value): void
+    {
+        Setting::set($key, $value);
+        Cache::forget(self::CACHE_KEY);
+    }
+
+    public function setMany(array $data): void
+    {
+        foreach ($data as $key => $value) {
+            Setting::set($key, $value);
+        }
+        Cache::forget(self::CACHE_KEY);
+    }
+}