Use a single app to host widgets

[devrsi0n]

Is your feature request related to a problem? Please describe.

Current widgets are built with dashboard app, but they have many different configurations. e.g. widgets have no header/footer, widgets have custom themes, and vercel always build the whole app even we have no change to widgets (duplicated/slow build).

To address these issues, we should extract all widget code to a new app.

Useful link:

• https://github.com/vercel/examples/blob/main/solutions/subdomain-auth/pages/api/auth/%5B...nextauth%5D.ts

Describe the solution you'd like

Describe alternatives you've considered

Additional context

Related to #433