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/views/agent-manager/AgentGroupDetail.tsx

@@ -278,7 +278,7 @@ export const AgentGroupDetail: React.FC<AgentGroupDetailProps> = ({
       </div>
 
       <Dialog open={Boolean(worktreeDialog)} onOpenChange={(open) => { if (!open) setWorktreeDialog(null); }}>
-        <DialogContent className="max-w-md" keyboardAvoid>
+        <DialogContent className="max-w-md">
           <DialogHeader>
             <DialogTitle>
               {worktreeDialog?.kind === 'remove' ? 'Remove worktree' : 'Remove other worktrees'}

packages/ui/src/components/views/agent-manager/AgentManagerSidebar.tsx

@@ -135,7 +135,7 @@ const AgentGroupItem: React.FC<AgentGroupItemProps> = ({ group, isSelected, isBu
       </div>
 
       <Dialog open={confirmOpen} onOpenChange={setConfirmOpen}>
-        <DialogContent className="max-w-md" keyboardAvoid>
+        <DialogContent className="max-w-md">
           <DialogHeader>
             <DialogTitle>Delete agent group</DialogTitle>
             <DialogDescription>