[FEATURE] Drawing Canvas

[adiagarwalrock]

Problem
Users are currently limited to text-based diagramming (Mermaid.js) and structured notes. There is no native way to freely sketch ideas, draw unstructured diagrams, or use a visual whiteboard for brainstorming within the platform.

Solution
Add a dedicated drawing canvas workspace (leveraging tldraw, which is already in the dependencies). This feature would allow users to create, save, and export free-form whiteboard sketches alongside their existing Mermaid diagrams and notes, fully integrated with the local SQLite persistence.