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/usage/UsagePage.tsx

@@ -148,7 +148,7 @@ export const UsagePage: 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 */}