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

database/migrations/2026_06_29_000001_create_settings_table.php

@@ -0,0 +1,23 @@
+<?php
+
+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::create('settings', function (Blueprint $table) {
+            $table->id();
+            $table->string('key')->unique();
+            $table->text('value')->nullable();
+            $table->timestamps();
+        });
+    }
+
+    public function down(): void
+    {
+        Schema::dropIfExists('settings');
+    }
+};