Add Node Tagging Functionality for Enhanced Search and Filtering

To improve the organization and search capabilities within OpenBrainMap, we should implement node tagging functionality. This feature will allow users to assign tags to nodes, making it easier to categorize and filter information within their mind maps.

No files, tests, or entry points are identified in the issue; start by surveying the repository to find how nodes are represented and where search and filtering are implemented. Define the tagging behavior and verify that users can assign tags to nodes, search by tags, and filter mind maps with them.

Written by the indexing model from the issue text.