Add dual authentication support and comprehensive provider examples

Core functionality:
- Add dual authentication support (username + email fallback)
- Enhance IMAPConnection to try username first, then email
- Add SOURCE_USERNAME and DEST_USERNAME configuration options
- Improve authentication error handling and logging

Configuration updates:
- Add Host Europe to Securehost.de migration examples
- Include authentication method explanations for various providers
- Add comprehensive provider-specific settings (Host Europe, Securehost.de, etc.)
- Document username vs email login methods with examples

Documentation updates:
- Add dual authentication section in README
- Include Host Europe and Securehost.de specific examples
- Expand troubleshooting section with authentication help
- Add provider-specific troubleshooting guidance
- Include migration best practices

Features:
- Automatic fallback from username to email authentication
- Enhanced logging showing which authentication method succeeded
- Support for various hosting providers and their login requirements
- Improved error messages for authentication failures

.env.template

@@ -20,6 +20,12 @@ SOURCE_IMAP_PORT=993
 # Example: user@yourdomain.de
 SOURCE_EMAIL=user@example-domain.de
 
+# SOURCE_USERNAME: Login username (if different from email)
+# Some providers use separate username for login (e.g. hosteurope123, mail_user)
+# Leave empty if login uses email address
+# Host Europe: often uses email account name or specific username
+SOURCE_USERNAME=
+
 # SOURCE_PASSWORD: Email account password
 # Use your Host Europe email password (NOT app password needed here)
 SOURCE_PASSWORD=your_hosteurope_password
@@ -29,24 +35,30 @@ SOURCE_PASSWORD=your_hosteurope_password
 SOURCE_IMAP_USE_SSL=True
 
 # ============================================================================
-# DESTINATION EMAIL ACCOUNT (migrating TO) - German Hosting Example
+# DESTINATION EMAIL ACCOUNT (migrating TO) - Securehost.de Example
 # ============================================================================
 
 # DEST_IMAP_SERVER: Destination IMAP server
-# Common formats: mail.yourdomain.de | imap.provider.de
-DEST_IMAP_SERVER=mail.yourdomain.de
+# Securehost.de: mail.securehost.de | Common: mail.yourdomain.de | imap.provider.de
+DEST_IMAP_SERVER=mail.securehost.de
 
 # DEST_IMAP_PORT: Destination IMAP port
 # 993 (SSL) | 143 (TLS/STARTTLS)
 DEST_IMAP_PORT=993
 
 # DEST_EMAIL: Your destination email address
-# Example: user@newdomain.de
-DEST_EMAIL=user@newdomain.de
+# Example: user@securehost.de
+DEST_EMAIL=user@securehost.de
+
+# DEST_USERNAME: Login username for destination (if different from email)
+# Some hosting providers use separate username (e.g. user123, cpanel_user)
+# Leave empty if login uses email address
+# Securehost.de: typically uses email or specific username
+DEST_USERNAME=
 
 # DEST_PASSWORD: Destination email password
-# Your hosting provider email password
-DEST_PASSWORD=your_destination_password
+# Your securehost.de email password
+DEST_PASSWORD=your_securehost_password
 
 # DEST_IMAP_USE_SSL: Use SSL for destination
 # True (recommended) | False
@@ -113,30 +125,60 @@ LOG_LEVEL=INFO
 # ./temp_emails (default) | /tmp/emails | C:\Temp\emails
 TEMP_DOWNLOAD_DIR=./temp_emails
 
+# ============================================================================
+# AUTHENTICATION EXAMPLES
+# ============================================================================
+
+# Email-based login (most common):
+# SOURCE_EMAIL=user@domain.de
+# SOURCE_USERNAME=
+# → Script tries: user@domain.de
+
+# Username-based login:
+# SOURCE_EMAIL=user@domain.de
+# SOURCE_USERNAME=mail_user123
+# → Script tries: mail_user123, fallback to user@domain.de
+
+# Host Europe specific:
+# SOURCE_USERNAME=email_account_name  (from Host Europe KIS)
+# SOURCE_USERNAME=                    (if using Easy-Mail-Login)
+
 # ============================================================================
 # PROVIDER-SPECIFIC EXAMPLES
 # ============================================================================
 
-# Host Europe Settings:
-# SOURCE_IMAP_SERVER=wpxxxxxxxx.mail.server-he.de
-# SOURCE_IMAP_PORT=993
-# SOURCE_IMAP_USE_SSL=True
-# Note: Replace 'wpxxxxxxxx' with your actual server name from Host Europe KIS
+# Host Europe to Securehost.de Migration:
+# SOURCE_IMAP_SERVER=wp123456.mail.server-he.de
+# SOURCE_EMAIL=user@olddomain.de
+# SOURCE_USERNAME=mail_account_he
+# DEST_IMAP_SERVER=mail.securehost.de
+# DEST_EMAIL=user@securehost.de
+# DEST_USERNAME=
 
-# Common German Hosting Providers:
-# Strato: imap.strato.de:993
-# 1und1/IONOS: imap.1und1.de:993
-# All-Inkl: mail.all-inkl.com:993
-# Hetzner: mail.your-server.de:993
+# Host Europe Settings:
+# Server format: wpxxxxxxxx.mail.server-he.de (get from KIS)
+# Port: 993 (SSL) or 143 (TLS)
+# Username: Check KIS for account name or use email if Easy-Mail-Login enabled
+
+# Securehost.de Settings:
+# Server: mail.securehost.de (typical format)
+# Port: 993 (SSL recommended)
+# Authentication: Usually email-based, check with provider
+
+# Other German Providers:
+# Strato: imap.strato.de:993 (email login)
+# 1und1/IONOS: imap.1und1.de:993 (username: username@domain)
+# All-Inkl: mail.all-inkl.com:993 (username varies)
+# Hetzner: mail.your-server.de:993 (email login)
 
 # Gmail (if needed):
-# Gmail: imap.gmail.com:993 (requires app password)
+# imap.gmail.com:993 (requires app password, email login)
 
 # ============================================================================
 # MIGRATION EXAMPLES
 # ============================================================================
 
-# Example 1: Host Europe to new hosting with organization
+# Example 1: Host Europe to Securehost.de with organization
 # IMPORT_FOLDER_NAME=HostEurope_Migration
 # Result: Clean separation in destination