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/Models/Setting.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+
+class Setting extends Model
+{
+    protected $fillable = ['key', 'value'];
+
+    public static function get(string $key, mixed $default = null): mixed
+    {
+        return static::where('key', $key)->value('value') ?? $default;
+    }
+
+    public static function set(string $key, mixed $value): void
+    {
+        static::updateOrCreate(['key' => $key], ['value' => $value]);
+    }
+
+    public static function allAsArray(): array
+    {
+        return static::pluck('value', 'key')->toArray();
+    }
+}