feat: iPad split layout for the Capacitor app (#2104)

* fix: open mobile model/agent panels on tablet-width Capacitor shells and keep composer taps from dismissing the keyboard

* feat: add iPadOS-style split layout to the Capacitor app

- classify the Capacitor shell as mobile in device detection so shared
  surfaces (draft starters, panels) stop falling into tablet branches
- add isIPadApp() and useOrientation() helpers
- iPad: persistent full-height sessions sidebar (mobile sessions surface
  inline), Changes/Files in a right sidebar with header shortcut toggles
- animate sidebar open/close like the desktop sidebars and add
  finger-sized drag-resize with persisted widths
- anchor the overflow menu and the usage/metadata popover next to their
  header buttons regardless of open sidebars

* fix: re-anchor metadata popover on layout shifts and untangle sidebar toggle updates

- recompute the iPad metadata popover anchor via a ResizeObserver on its
  wrapper so sidebar toggles/resizes while it is open cannot leave it
  misplaced
- move the portrait right-panel close out of the setIpadSidebarOpen
  updater into plain sequential state updates

packages/ui/src/components/chat/MobileModelButton.tsx

@@ -22,6 +22,16 @@ export const MobileModelButton: React.FC<MobileModelButtonProps> = ({ onOpenMode
         <button
             type="button"
             onClick={onOpenModel}
+            // Same guard as PermissionAutoAcceptButton/MobileAgentButton: block
+            // the focus transfer so the tap doesn't dismiss the keyboard. With
+            // interactive-widget=resizes-content (Android), the keyboard-close
+            // relayout moves this button mid-tap and the click never lands.
+            onMouseDown={(event) => event.preventDefault()}
+            onPointerDownCapture={(event) => {
+                if (event.pointerType === 'touch') {
+                    event.preventDefault();
+                }
+            }}
             className={cn(
                 'inline-flex min-w-0 items-stretch',
                 'rounded-lg',