getItemIcon/getIconColor/ItemThumbnail move out of files_view.dart into
widgets/item_icon.dart so other screens (the share-upload destination
picker, in a follow-up commit) can reuse the same file/folder icon
treatment instead of re-deriving it.
The Files controls bar's "this scrolls" hint is now a one-shot nudge
(scrolls right then back once on first appearance) instead of a
persistent chevron badge or edge fade - both prior attempts blended into
the row's sparse icon content too easily to actually register.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
