refactor: remove webmail interface, focus on core mail storage functionality

- Remove obsolete CouchDB design documents (webmail.json, dashboard.json)
- Clean up webmail-related code from couch/couch.go (WebmailViews, CreateWebmailViews, etc.)
- Update documentation to focus on core mail-to-CouchDB storage functionality
- Add Future Plans section describing planned webmail viewer as separate component
- Apply go fmt formatting and ensure code quality standards
- Update test documentation to show raw CouchDB API access patterns
- Remove compiled binary from repository

This refactor simplifies the codebase to focus on its core purpose: efficiently
backing up emails from IMAP to CouchDB. The webmail interface will be developed
as a separate, optional component to maintain clean separation of concerns.

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

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

test/test-incremental-sync.sh

@@ -81,7 +81,17 @@ add_new_messages() {
 
 import imaplib
 import time
-from test.populate_greenmail import create_simple_message
+import sys
+import os
+
+# Add the test directory to Python path to enable imports
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+
+import importlib.util
+spec = importlib.util.spec_from_file_location("populate_greenmail", "populate-greenmail.py")
+populate_greenmail = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(populate_greenmail)
+create_simple_message = populate_greenmail.create_simple_message
 
 def add_new_messages():
     """Add new messages to test incremental sync"""