Add keyboard shortcut to stop running tasks in Desktop

In the CLI and VS Code extension, you can press Escape to stop Claude mid-execution. In Claude Code Desktop, the only way to stop a running task is to click the stop button — there's no keyboard shortcut.

The keybinding system's Task context only has task:background (ctrl+b). There's no task:stop or task:cancel action to bind to.

This is a problem when you need to stop Claude quickly (e.g., it's about to commit or push something you don't want) and you don't want to take your hands off the keyboard.

Requested change: Add a task:stop action to the Task keybinding context, ideally defaulting to escape to match CLI behavior.