Merge backend into src-tauri

[danielvschoor]

Merge the back end API into src-tauri and call it via invoke instead of doing GET/POST.

Pros:

• No localhost server
• Reduce dependencies by around half (faster builds, less maintenance)
• More secure

Cons:

• Back end and front end are tightly wound. Can't swap back ends. Can't easily migrate to another framework, like Electron