Add Google Drive authorization processing and error handling templates

- Implemented `google_drive_callback.html` for processing Google Drive authorization, including UI for success and error states.
- Added JavaScript functionality for exchanging authorization codes, saving settings, and handling folder selection.
- Created `google_drive_callback_error.html` to display error messages during the authorization process.

.env.demo

@@ -14,7 +14,7 @@ ADMIN_PASSWORD=your_secure_password
 
 # **OpenID Connect/Authentik Settings**
 AUTHENTIK_CLIENT_ID=<yourAuthentikAppClientID>
-AUTHENTIK_CLIENT_SECRET=<yourAuthentikAppClientSecret>
+AUTHENTIK_CLIENT_SECRET=<yourAuthentikClientSecret>
 AUTHENTIK_CONFIG_URL=<ConfigUrlOfYourApp, e.g. https://authentik.example.com/application/o/docuelevate/.well-known/openid-configuration>
 
 # **AI/ML Services**
@@ -81,10 +81,17 @@ DROPBOX_REFRESH_TOKEN=<DROPBOX_REFRESH_TOKEN>
 DROPBOX_FOLDER="/Documents/Uploads"
 
 # Google Drive
+# Service Account Method:
 GOOGLE_DRIVE_CREDENTIALS_JSON={"type":"service_account","project_id":"your-project","private_key_id":"key-id","private_key":"-----BEGIN PRIVATE KEY-----\nYOUR_PRIVATE_KEY\n-----END PRIVATE KEY-----\n","client_email":"service-account@project.iam.gserviceaccount.com","client_id":"client-id","auth_uri":"https://accounts.google.com/o/oauth2/auth","token_uri":"https://oauth2.googleapis.com/token","auth_provider_x509_cert_url":"https://www.googleapis.com/oauth2/v1/certs","client_x509_cert_url":"https://www.googleapis.com/robot/v1/metadata/x509/service-account%40project.iam.gserviceaccount.com"}
 GOOGLE_DRIVE_FOLDER_ID=<YOUR_FOLDER_ID>
 GOOGLE_DRIVE_DELEGATE_TO=<OPTIONAL_USER_EMAIL>
 
+# OAuth Method (Alternative):
+GOOGLE_DRIVE_USE_OAUTH=false  # Set to true to use OAuth instead of service account
+GOOGLE_DRIVE_CLIENT_ID=your-oauth-client-id  # Required for OAuth method
+GOOGLE_DRIVE_CLIENT_SECRET=your-oauth-client-secret  # Required for OAuth method
+GOOGLE_DRIVE_REFRESH_TOKEN=your-oauth-refresh-token  # Required for OAuth method
+
 # OneDrive
 ONEDRIVE_CLIENT_ID=your-client-id
 ONEDRIVE_CLIENT_SECRET=your-client-secret