refactor(frontend): button and icon components on tailwind-variants

[Kadrian]

Second step of the emotion → tailwind migration: src/js/button/* and src/js/icon/FaIcon are emotion-free.

• font sizes: only tailwind's own (text-xs…text-2xl), anything else is a literal (text-[11px]). No custom size tokens: tv's class merger classifies unknown text-* names as colours and would drop them next to a colour class, literals are recognised as sizes
• BasicButton, IconButton, FaIcon: prop flags → tv variants. Where several flags conflict (tiny/small/large, colour flags) the variant order encodes the old precedence (later wins), noted inline; FaIcon's colour priority stays a small function
• PrimaryButton, TransparentButton, DestroyButton, SelectFileButton, BadgeToggleButton, PreviewButton, QueryResultHistoryButton, DownloadButton: real components, className merges through tv. DownloadButton reads the filetype-* colours as CSS vars instead of useTheme
• Icon (FaIcon without fa-fw) is gone: FaIcon forces width: initial anyway, so the 4 consumers use FaIcon
• --animate-spin-fast token keeps the 0.5s spinner
• text-xs … text-2xl are font-size only now (--text-*--line-height: initial): the emotion theme never set a line-height, so migrated components would otherwise shift by a few px
• emotion no longer strips non-DOM props before <button>, SxSelectFileButton gets a shouldForwardProp until its file is migrated
• FaIcon's @ts-ignore on ref is gone

Remaining styled(IconButton) / styled(FaIcon) overrides in other files keep working: emotion classes are unlayered and beat the utilities.