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/SettingsView.tsx

@@ -604,7 +604,7 @@ export const SettingsView: React.FC<SettingsViewProps> = ({ onClose, forceMobile
         // No sidebar available; fall back to direct content.
         const fallback = renderPageContent(settingsSlug);
         return (
-          <div className="flex-1 min-h-0 overflow-hidden bg-background" data-keyboard-avoid="true">
+          <div className="flex-1 min-h-0 overflow-hidden bg-background">
             <ErrorBoundary>{fallback}</ErrorBoundary>
           </div>
         );
@@ -622,7 +622,7 @@ export const SettingsView: React.FC<SettingsViewProps> = ({ onClose, forceMobile
     const content = renderPageContent(settingsSlug);
 
     return (
-      <div className="flex-1 min-h-0 overflow-hidden bg-background" data-keyboard-avoid="true">
+      <div className="flex-1 min-h-0 overflow-hidden bg-background">
         <ErrorBoundary>{content}</ErrorBoundary>
       </div>
     );