refactor: drop mobile keyboard/viewport hacks, rely on browser

Remove body position:fixed lock, visualViewport listener in MainLayout
(keyboard-inset heuristics, scroll-to-zero lock, focusin/focusout RAF),
[data-keyboard-avoid-active] translateY rule, and related CSS vars
(--oc-keyboard-inset, --oc-keyboard-avoid-offset, --oc-keyboard-home-indicator,
--oc-visual-viewport-offset-top). Strip data-keyboard-avoid* attrs and
keyboardAvoid props from Dialog/ScrollableOverlay and consumers. Drop
isKeyboardOpen from useUIStore.

With body unlocked and the layout plain flex-col (h-dvh), the browser
shrinks the viewport on keyboard open naturally — composer sits above
the keyboard, header stops lagging, input no longer jitters.

FilesView.nudgeEditorSelectionAboveKeyboard now derives the occluded
bottom locally from visualViewport + documentElement.clientHeight.

packages/ui/src/components/session/SessionFolderItem.tsx

@@ -178,7 +178,7 @@ const SessionFolderItemBase = <TSessionNode,>({
           {renaming ? (
             <form
               className="flex min-w-0 flex-1 items-center gap-1"
-              data-keyboard-avoid="true"
+
               onPointerDown={(event) => event.stopPropagation()}
               onMouseDown={(event) => event.stopPropagation()}
               onSubmit={(event) => {