feature: make sure the app works as a SPA

[nsarrazin]

This is a prerequisite for bundling chat-ui as part of an electron/tauri or capacitor app. We need to move all server logic to routes/api and make sure the rest of the app can work as a SPA.

The following things need to be done first:

• refactor: replace all server-side load functions by universal load functions #1699
• refactor: replace all form actions by API endpoints #1698