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/stores/useUIStore.ts

@@ -482,7 +482,6 @@ interface UIStore {
   pendingFileNavigation: PendingFileNavigation | null;
   pendingFileFocusPath: string | null;
   isMobile: boolean;
-  isKeyboardOpen: boolean;
   isQuickOpenOpen: boolean;
   isCommandPaletteOpen: boolean;
   isHelpDialogOpen: boolean;
@@ -637,7 +636,6 @@ interface UIStore {
   setPadding: (size: number) => void;
   setCornerRadius: (radius: number) => void;
   setInputBarOffset: (offset: number) => void;
-  setKeyboardOpen: (open: boolean) => void;
   applyTypography: () => void;
   applyPadding: () => void;
   updateProportionalSidebarWidths: () => void;
@@ -724,7 +722,6 @@ export const useUIStore = create<UIStore>()(
         pendingFileNavigation: null,
         pendingFileFocusPath: null,
         isMobile: false,
-        isKeyboardOpen: false,
         isQuickOpenOpen: false,
         isCommandPaletteOpen: false,
         isHelpDialogOpen: false,
@@ -1470,10 +1467,6 @@ export const useUIStore = create<UIStore>()(
           set({ inputBarOffset: offset });
         },
 
-        setKeyboardOpen: (open) => {
-          set({ isKeyboardOpen: open });
-        },
-
         toggleFavoriteModel: (providerID, modelID) => {
           set((state) => {
             const exists = state.favoriteModels.some(