…& Manage Content

Replace Browse Mods / Manage Mods with a content-type picker that lets you browse or manage mods,
resource packs, or shader packs through the same search/install/update
pipeline. Introduces a ContentType enum carrying folder name, file
extension, and Modrinth/CurseForge search parameters per kind.

Relocates the per-instance version/loader filter from a button inside
Manage Mods to the Manage Content picker, since it's shared across all
three content types rather than mod-specific. The loader half of that
filter is now ignored for resource packs and shader packs everywhere
(search, dependency install, update checks), and auto-fills from the
instance's own version/loader the first time it's opened.

InstalledModAdapter generalized from hardcoded .jar scanning to any
ContentType's extension; icon/name extraction already degraded
gracefully for non-mod content so needed no changes there.

Browse Content and Manage Content's + search button previously opened
unfiltered until the user manually opened the filter dialog and hit
Apply. Both now fall back to the instance's own detected version/loader
(same InstanceVersionResolver logic the filter dialog already uses) when
no filter has been saved yet, so results are relevant immediately.

Also fixed the Browse/Manage Content picker dialog's corners looking
mismatched — the layout draws its own rounded card, but the dialog
window's default background was still showing through behind it. Window
background is now transparent so only the card's rounded corners show.