feat: implement Go-based mail2couch with working IMAP and CouchDB integration

- Add configuration system with automatic file discovery (current dir, config subdir, user home, XDG config)
- Implement IMAP client with TLS connection, authentication, and mailbox listing
- Add CouchDB integration with database creation and document storage
- Support folder filtering (include/exclude) and date filtering (since parameter)
- Include duplicate detection to prevent re-storing existing messages
- Add comprehensive error handling and logging throughout
- Structure code in clean packages: config, mail, couch
- Application currently uses placeholder messages to test the storage pipeline
- Ready for real IMAP message parsing implementation

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

config.json

@@ -0,0 +1,42 @@
+{
+  "couchDb": {
+    "url": "http://localhost:5984",
+    "user": "admin",
+    "password": "password",
+    "database": "mail_backup"
+  },
+  "mailSources": [
+    {
+      "name": "Personal Gmail",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "imap.gmail.com",
+      "port": 993,
+      "user": "your-email@gmail.com",
+      "password": "your-app-password",
+      "sync": true,
+      "folderFilter": {
+        "include": ["INBOX", "Sent"],
+        "exclude": ["Spam", "Trash"]
+      },
+      "messageFilter": {
+        "since": "2024-01-01"
+      }
+    },
+    {
+      "name": "Work Account",
+      "enabled": true,
+      "protocol": "imap",
+      "host": "imap.work.com",
+      "port": 993,
+      "user": "user@work.com",
+      "password": "password",
+      "sync": true,
+      "folderFilter": {
+        "include": [], 
+        "exclude": []
+      },
+      "messageFilter": {}
+    }
+  ]
+}