Open the model picker without the keyboard on phones

[Tryanks]

The model picker's popover tracked the search input's focus handle, and
PopoverState focuses the tracked handle the moment it opens, so on a phone
the sheet came up with the software keyboard over the list it means to be
tapped. The tracked focus is now attached only off-phone: a desktop window
of any width still types into the search as the picker opens, while a
mobile build lets the popover keep its own focus, so Escape and Back
still close it and a tap on the search field asks for the keyboard.

The composer's own focus checks read !compact && !is_mobile. Compact is
already is_mobile && narrow, so the width half was redundant and read as
if the window width took part in the keyboard decision. They now check the
build alone, and ChatView::focus_composer no longer pre-empts the composer
with its own compact check.

model_picker_focuses_its_search_only_off_phone opens the picker in a wide
desktop window, a narrow desktop window and on a phone: the search is
focused in the first two and unfocused on the phone, and Escape closes the
picker in all three.