Fix window keydown listener leak in EventForm

[thedhanawada]

Bug

EventForm.js (line ~308-316) adds a window keydown listener but the reference is not tracked by the cleanup() method. When the form is destroyed and recreated, listeners accumulate.

Fix

Store the listener reference and remove it in cleanup() / disconnectedCallback().

[thedhanawada]

Resolved. The keydown listener is now guarded by _keydownListenerAdded flag to prevent duplicate registration, checks hasAttribute('open') so it's inert when modal is closed, and is properly cleaned up in unmount(). No leak remains.