feat: Add DatabaseManager and LinkPlayer components, implement authentication and linking logic

- Created DatabaseManager component for managing database access via phpMyAdmin.
- Developed LinkPlayer component to link Discord accounts with game characters, including user authentication and error handling.
- Added mock data files for players, organizations, and projects to handle backend unavailability.
- Implemented AuthService for managing user authentication and session checks.
- Created DatabaseService to fetch and manage player, organization, and project data with fallback to mock data.
- Added HTML page for handling authentication unavailability.
- Developed a test script for validating Docker setup and required files.

pages/DatabaseManager.tsx

@@ -0,0 +1,25 @@
+import React from 'react';
+
+const DatabaseManager: React.FC = () => {
+  return (
+    <div className="flex flex-col items-center justify-center h-full text-center p-10 animate-in fade-in">
+      <div className="bg-surface border border-border rounded-xl p-8 max-w-md shadow-card">
+        <h1 className="text-2xl font-bold mb-4 text-textMain">Datenbank-Manager</h1>
+        <p className="text-textMuted mb-6 leading-relaxed">
+          Die Datenbank wird über eine externe MySQL-Instanz verwaltet. 
+          Für administrative Eingriffe nutze bitte das phpMyAdmin Interface.
+        </p>
+        <a 
+            href="http://localhost:8081" 
+            target="_blank" 
+            rel="noopener noreferrer"
+            className="inline-flex items-center justify-center px-6 py-3 bg-accentInfo text-white rounded-lg hover:bg-accentInfo/90 transition-colors font-medium shadow-glow"
+        >
+            phpMyAdmin öffnen
+        </a>
+      </div>
+    </div>
+  );
+};
+
+export default DatabaseManager;