In dev environment, this PR allows to proxy to whatever backend without CORS issue by specifying the backend URL in `.env.local` variable.
I believe this would ease contribution by frontend developpers that would only have to run the unstable docker to work on the frontend without need for all the go toolchain to build the whole backend to have a cors enabled backend.
Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/vikunja/pulls/3069
Co-authored-by: Marc <marc88@free.fr>
Co-committed-by: Marc <marc88@free.fr>
