refactor: simplify mobile keyboard handling

Removed fragile global keyboard viewport manager
Restored native mobile viewport behavior
Fixed textarea content shifting outside inputs

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((state) => state.isKeyboardOpen === open ? state : { isKeyboardOpen: open });
-        },
-
         toggleFavoriteModel: (providerID, modelID) => {
           set((state) => {
             const exists = state.favoriteModels.some(