fix(app): rest the iPhone composer at the screen's bottom clearance (#8823)

* fix(app): rest the iPhone composer at the screen's bottom clearance

The composer sat 50pt above the bottom edge on every iPhone: the dock
padded the rectangular home-indicator safe area (34pt) and the composer
stacked another 16pt under it. On iPhone Duo that safe area is also
taller than needed; iOS 27.1's bottom-bar layout guide, which follows
the screen's corners, rests 26pt from the edge.

The native window-layout probe now publishes that clearance, and
WindowLayoutProvider overrides the safe-area context's bottom with it,
so every useSafeAreaInsets() consumer reads one bottom value and null
falls back to the system inset. The docked composer applies
max(bottom, 16) once and keeps 16pt above the keyboard through the
translate view's transform; the gap is not animated as padding because
layout updates through the keyboard transition are expensive on Android
Fabric.

Co-authored-by: Elsa Granger <zeyugao@outlook.com>

* fix(app): keep the Import session gap under the wide new workspace composer

The composer's own bottom spacing used to separate it from the Import
session pill on the wide form. With that spacing moved to the dock, the
centered slot padded below the pill instead, so the pill touched the
composer. The pill now owns its gap above.

Co-authored-by: Elsa Granger <zeyugao@outlook.com>

---------

Co-authored-by: Elsa Granger <zeyugao@outlook.com>

docs/design.md

@@ -169,6 +169,8 @@ Electron window controls are top-corner obstructions, not a compact-layout condi
 
 Windows and Linux controls are fixed window chrome, outside scrolling header content. A tab rail that reaches them ends at their obstruction and shows the shared overflow fade. On macOS, the Explorer toggle occupies a fixed top-right window slot so opening and closing Explorer does not move the pointer target.
 
+Each window edge has one safe-area owner. `WindowLayoutProvider` publishes the bottom inset that every surface reads through `useSafeAreaInsets()`: on iOS 27.1 and later it is UIKit's bottom-bar clearance, which refines the rectangular home-indicator inset to the screen's corners, and elsewhere the system inset. Surfaces apply that value once and never stack their own spacing under it; `WindowLayoutSurface` owns the horizontal insets the same way.
+
 A new list+detail feature copies the settings shell. A new workspace-shaped feature copies the workspace shell. Inventing a third shape happens in design review, not in a PR.
 
 ---

docs/floating-panels.md

@@ -172,6 +172,11 @@ slide under the header on open and out from under it on close. Do not swap the
 dock to keyboard padding at rest. Changing its height creates either a blank
 band or a paused transition when the stream list updates its viewport.
 
+The docked composer rests at the larger of the safe-area bottom and 16pt. With
+the keyboard open it keeps 16pt above the keyboard through the translate view's
+`keyboardGap`, which rises further than the keyboard's safe-area overlap. Do not
+grow the composer's bottom padding with the keyboard instead.
+
 Do not animate a layout prop through the keyboard transition. On Android
 Fabric, each animated layout update clones the shadow tree and runs Yoga over
 the dock subtree. This measured about 10 ms per frame on the emulator and the