Cannot exit fullscreen gadget on mobile — requires Escape key

## Problem

When a gadget is in fullscreen mode, exiting requires pressing the **Escape key**. On mobile devices, there is no Escape key available (mobile keyboards don't include it), making it **impossible to exit fullscreen mode** on mobile.

This effectively traps the user in the fullscreen gadget view with no way to get back to the normal workspace.

## Expected behavior

There should be a **visible on-screen button** (e.g. a close/exit "×" or a back button) to exit fullscreen gadget mode, so it works on devices without a physical keyboard.

## Context

- This is a mobile-specific issue — desktop users can press Escape as expected.
- Related to #64 (general mobile responsiveness of the workspace view).
- The fix should be straightforward: add a visible exit affordance in addition to the Escape key handler.

Start at the fullscreen gadget view and locate the existing Escape-key handler. Add a visible exit control that works on mobile while keeping desktop Escape behavior, then verify that users can return to the normal workspace from fullscreen mode on a mobile-sized view.

Written by the indexing model from the issue text.