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/sections/skills/SkillsPage.tsx

@@ -340,7 +340,7 @@ const SkillsInstalledPage: React.FC = () => {
   }
 
   return (
-    <ScrollableOverlay keyboardAvoid outerClassName="h-full" className="w-full">
+    <ScrollableOverlay outerClassName="h-full" className="w-full">
       <div className="mx-auto w-full max-w-3xl p-3 sm:p-6 sm:pt-8">
 
         {/* Header */}
@@ -558,7 +558,7 @@ const SkillsInstalledPage: React.FC = () => {
         setIsFileDialogOpen(open);
         if (!open) setEditingFilePath(null);
       }}>
-        <DialogContent className="max-w-3xl max-h-[85vh] flex flex-col" keyboardAvoid>
+        <DialogContent className="max-w-3xl max-h-[85vh] flex flex-col">
           <DialogHeader className="flex-shrink-0">
             <DialogTitle>{editingFilePath ? 'Edit Supporting File' : 'Add Supporting File'}</DialogTitle>
             <DialogDescription>

packages/ui/src/components/sections/skills/catalog/AddCatalogDialog.tsx

@@ -230,7 +230,7 @@ export const AddCatalogDialog: React.FC<AddCatalogDialogProps> = ({ open, onOpen
 
   return (
     <Dialog open={open} onOpenChange={onOpenChange}>
-      <DialogContent className="max-w-xl" keyboardAvoid>
+      <DialogContent className="max-w-xl">
         <DialogHeader>
           <DialogTitle>Add skills catalog</DialogTitle>
           <DialogDescription>

packages/ui/src/components/sections/skills/catalog/InstallSkillDialog.tsx

@@ -156,7 +156,7 @@ export const InstallSkillDialog: React.FC<InstallSkillDialogProps> = ({ open, on
   return (
     <>
       <Dialog open={open} onOpenChange={onOpenChange}>
-        <DialogContent className="max-w-md" keyboardAvoid>
+        <DialogContent className="max-w-md">
           <DialogHeader>
             <DialogTitle>Install skill</DialogTitle>
             <DialogDescription>

packages/ui/src/components/sections/skills/catalog/SkillsCatalogPage.tsx

@@ -144,7 +144,7 @@ export const SkillsCatalogPage: React.FC<SkillsCatalogPageProps> = ({ mode, onMo
   };
 
   return (
-    <ScrollableOverlay keyboardAvoid outerClassName="h-full" className="w-full">
+    <ScrollableOverlay outerClassName="h-full" className="w-full">
       <div className="mx-auto w-full max-w-3xl p-3 sm:p-6 sm:pt-8">
 
         {/* Header */}