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.

Dockerfile.backend

@@ -0,0 +1,20 @@
+# Backend Dockerfile
+FROM node:20-alpine
+
+# Set working directory
+WORKDIR /app
+
+# Copy package files
+COPY backend/package.json .
+
+# Install dependencies
+RUN npm install
+
+# Copy source files
+COPY backend/ /app/
+
+# Expose port
+EXPOSE 3000
+
+# Start the application
+CMD ["node", "server.js"]