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/git/CommitSection.tsx

@@ -53,7 +53,7 @@ export const CommitSection: React.FC<CommitSectionProps> = ({
   const contentClassName = 'flex flex-col gap-3 px-0 pt-1 pb-3';
 
   return (
-    <section className={containerClassName} data-keyboard-avoid="true">
+    <section className={containerClassName}>
       <div className={headerClassName}>
         <h3 className="typography-ui-header font-semibold text-foreground">Commit</h3>
       </div>