Complete Google authentication implementation with documentation

Co-authored-by: clayschaad <11883505+clayschaad@users.noreply.github.com>

authentication-setup.md

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+# Authentication Setup Guide
+
+## Overview
+The ShiftScheduler application now includes Google OAuth authentication with configurable authorized email addresses. Only users with emails listed in the configuration can access the application.
+
+## Setup Instructions
+
+### 1. Create Google OAuth Application
+1. Go to the [Google Cloud Console](https://console.cloud.google.com/)
+2. Create a new project or select an existing one
+3. Enable the Google+ API
+4. Go to "Credentials" and create OAuth 2.0 Client IDs
+5. Set the authorized redirect URI to: `http://localhost:5000/api/auth/callback` (for development)
+6. For production, use your domain: `https://yourdomain.com/api/auth/callback`
+
+### 2. Configure Application
+Edit `Server/appsettings.json` and update the authentication section:
+
+```json
+{
+  "Authentication": {
+    "Google": {
+      "ClientId": "your-google-client-id.apps.googleusercontent.com",
+      "ClientSecret": "your-google-client-secret"
+    },
+    "AuthorizedEmails": [
+      "user1@gmail.com",
+      "user2@example.com"
+    ]
+  }
+}
+```
+
+### 3. For Production
+For production deployment, consider using environment variables or Azure Key Vault:
+- `Authentication__Google__ClientId`
+- `Authentication__Google__ClientSecret`
+- `Authentication__AuthorizedEmails__0`, `Authentication__AuthorizedEmails__1`, etc.
+
+## How It Works
+
+### Authentication Flow
+1. Unauthenticated users see a login screen
+2. Clicking "Sign in with Google" redirects to Google OAuth
+3. After successful Google authentication, the application checks if the user's email is in the authorized list
+4. Authorized users are redirected to the main application
+5. Unauthorized users are redirected back with an error message
+
+### API Security
+- All API endpoints require authentication (`[Authorize]` attribute)
+- Only users with emails in the `AuthorizedEmails` list can access the API
+- Unauthenticated requests return a 302 redirect to login
+
+### User Interface
+- **Login Screen**: Clean, centered login form with Google sign-in button
+- **Authenticated Header**: Shows user email and sign-out button
+- **Main Application**: Normal shift scheduler functionality for authenticated users
+
+## Testing
+To test the authentication:
+1. Configure Google OAuth credentials as described above
+2. Add your email to the `AuthorizedEmails` list
+3. Start the application: `dotnet run` from the Server directory
+4. Navigate to `http://localhost:5000`
+5. Click "Sign in with Google" and complete the OAuth flow
+
+## Security Features
+- **Email-based Authorization**: Only specified emails can access the application
+- **Secure API Endpoints**: All shift management APIs require authentication
+- **Session Management**: Proper login/logout functionality
+- **OAuth Integration**: Uses Google's secure OAuth 2.0 flow