expose app and preview on landing page

[maxheld83]

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[maxheld83]

this would address #321

[maxheld83]

the UI with this would look better if we also had DOI-matching highlighting inside the DOI text input field as per https://github.com/subugoe/biblids/issues/118.
Then it becomes easy to have only inputs in the sidebar, and outputs in the main frame.
Alas, implementing https://github.com/subugoe/biblids/issues/118 is a bit of a sideshow, so not happening anytime soon probably.

Should still work without https://github.com/subugoe/biblids/issues/118, though then entry and syntax highlighting have to happen one below the other, which can be visually confusing.