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/index.css

@@ -223,10 +223,6 @@ textarea[data-chat-input="true"]:focus-visible {
   -webkit-appearance: none;
 }
 
-[data-keyboard-avoid-active="true"] {
-  transform: translateY(calc(-1 * var(--oc-keyboard-avoid-offset, 0px)));
-}
-
 /* Scroll shadow (HeroUI) fallback styling to ensure visible gradients without the Tailwind plugin */
 [data-scroll-shadow="true"] {
   --scroll-shadow-size: var(--scroll-shadow-size, 48px);