Fix: set Content-type to JSON

[arminfelder]

Sets the content-type to application/json

solves issues with modsecurity

Fixes #2600

[jhaals]

Thanks for this @arminfelder!
Just needs a small format to be able to merge :)
See https://github.com/jhaals/yopass/actions/runs/12187426977/job/34309743381?pr=2599

[arminfelder]

@jhaals I did a reformat and rebase, should be fine now

[jhaals]

Thanks for the rebase. I was just about to merge this but I realized that the OPTIONS call fails for the preview build. It's probably the case that the /secret endpoint need to support OPTIONS as well https://github.com/jhaals/yopass/blob/master/pkg/server/server.go#L150

[arminfelder]

@jhaals I just added implementations for OPTIONS method calls + a config option to override the default '*'
the question is, if we should do this together in this pull request or split it up