fix: resolve document ID conflicts with URL encoding

- Fixed document ID conflicts caused by unencoded slashes in mailbox names
- Added URL encoding for all document IDs used in CouchDB REST API calls
- Mailbox names with slashes (e.g., 'Work/Projects') now create proper document IDs
- Resolves issue where 'Work/Projects_1' was incorrectly stored as document 'Work' with attachment 'Projects_1'
- Added urlencoding dependency for proper URL-safe document ID handling

All messages now store successfully without conflicts across all mailboxes.

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

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

rust/Cargo.toml

@@ -52,6 +52,9 @@ dirs = "5.0"
 # Pattern matching for folder filters
 glob = "0.3"
 
+# URL encoding for document IDs
+urlencoding = "2.1"
+
 [dev-dependencies]
 # Testing utilities
 tokio-test = "0.4"