feat(themes): set different default banner per theme

app/Config/Fediverse.php

app/Config/Images.php

app/Controllers/WebmanifestController.php

app/Entities/Podcast.php

modules/Admin/Controllers/PodcastController.php

modules/Fediverse/Config/Fediverse.php

public/media/castopod-avatar.jpg

public/media/castopod-avatar_medium.webp

public/media/castopod-avatar_thumbnail.webp

public/media/castopod-avatar_tiny.webp

public/media/castopod-banner-amber.jpg

public/media/castopod-banner-amber_federation.jpg

public/media/castopod-banner-amber_medium.webp

public/media/castopod-banner-amber_small.webp

public/media/castopod-banner-crimson.jpg

public/media/castopod-banner-crimson_federation.jpg

public/media/castopod-banner-crimson_medium.webp

public/media/castopod-banner-crimson_small.webp

public/media/castopod-banner-jacaranda.jpg

public/media/castopod-banner-jacaranda_federation.jpg