feat: Add duplicate detection, folder management, and enhanced UI

- Implement smart duplicate email detection using Message-ID and fallback signatures
- Add automatic folder creation with existing folder detection and reuse
- Enhance terminal output with colors, progress bars, and professional formatting
- Replace import folder functionality with original folder structure preservation
- Add comprehensive statistics tracking (duplicates, folder creation, etc.)
- Improve error handling with graceful date format fallbacks
- Add universal terminal compatibility with line-based formatting
- Update documentation and configuration files
- Provide clear user feedback for all migration decisions

Migration now intelligently skips duplicates, preserves folder structure,
and provides detailed feedback on what was migrated vs. what was skipped.

.env.template

@@ -67,20 +67,16 @@ DEST_PASSWORD=your_securehost_password
 DEST_IMAP_USE_SSL=True
 
 # ============================================================================
-# IMPORT FOLDER CONFIGURATION
+# FOLDER STRUCTURE
 # ============================================================================
-
-# IMPORT_FOLDER_NAME: Where to organize imported emails
 #
-# Set to folder name: Creates organized subfolders
-#   "Imported" → Imported/INBOX, Imported/Sent, Imported/Drafts
-#   "Migration" → Migration/INBOX, Migration/Sent, etc.
+# This script preserves the original folder structure from your source account:
+# - INBOX emails go to INBOX
+# - Sent emails go to Sent folder
+# - Custom folders are recreated on the destination
 #
-# Leave empty: All emails go to main INBOX
-#   "" → All emails regardless of source folder → INBOX
+# Folders that do not exist on the destination will be created automatically.
 #
-# Examples: Imported | Migration_2024 | HostEurope_Backup | (empty)
-IMPORT_FOLDER_NAME=Imported
 
 # ============================================================================
 # FOLDER FILTERING