Adds drag-over and drag-leave styling, drop with no files, multiple
invalid files, mixed valid+invalid files, non-Enter keydown ignore,
window-level dragenter/dragleave with and without 'Files' types,
counter underflow guard.
16 tests, +9 covered branches.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Drop hint + accepted types render, default no-progress state, invalid
MIME-type rejection, valid PDF acceptance, no-files early return,
click + Enter open the file input, multi-file accept whitelist
attributes.
8 tests covering ~25 of DropZone's 46 branches.
Refs #496.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
