Add clear separation between main and renderer processes

[cristianoventura]

We'd like to separate the code for the main process and renderer process into distinct modules or directories to establish a clear separation of concerns.

The main process should only contain code related to the application's lifecycle, system-level operations, and IPC setup. (#378 is responsible for organizing the main process).

The renderer process should only contain code related to rendering the UI, frontend logic, and interaction with the user (React code).

Following patterns adopted by other open-source Electron projects, create a clear folder structure to reflect the separation.