iOS/iPadOS 26: update Item component and screen

[yexingsha]

Aside from the common changes for list rows, Item rows also need the following changes:

• Reduce the gap between item type icon and title, and between title and attachment by 4px.
• Set the subtitle to subhead.
• On iPad, selected rows now use an background with 12px horizontal inset. This requires the right padding to be increased by 4px, and the selection button to be moved 8px to the right.

Regarding the screen:

• On iOS/iPadOS 26, sync and download status is displayed as subtitle in the top toolbar (see: Mail).
  • On iOS, this makes room for the search bar.
  • On iPadOS, this means the collection title needs to stay visible, which will cause the search bar to be collapsed at smaller widths (the system seems to handle it pretty well, but should be tested after this change).
• The filter symbol needs to use the version without borders (see HIG). When there is an active filter, the symbol should not change, but the button should use a "Preferred" state (can't find specifics in Apple's documentation, but it's what Mail does.)
• On entering Select mode:
  • On iOS, the Add button is hidden, the Back button becomes Select All, and the Select button becomes Checkmark. In the bottom toolbar, the left button becomes Share, and the right button becomes an overflow button, containing the rest of the available actions in a menu (see: Files).
  • On iPadOS, the Add button becomes Select All, and the Select button becomes Checkmark. The buttons in the bottom toolbar should be grouped together, and their color should stay as default regardless of whether there are selected items.