Summary

• Add Escape key handling in the desktop compose text field to cancel the current compose context (editing, quoting, forwarding, reporting)
• When editing: Escape clears compose state entirely (same as clicking the X button)
• When quoting/forwarding/reporting: Escape removes the context but preserves typed text
• When no context is active: Escape is not consumed and propagates normally

Closes #6612

Test plan

• Start editing a message, press Escape -edit cancelled, compose area cleared
• Start replying to a message (quote), press Escape - quote removed, typed text preserved
• Forward a message, press Escape -forward context removed
• Report a message, press Escape -report context removed
• Type in empty compose area (no context), press Escape - nothing happens, event propagates
• Verify Enter, Shift+Enter, Up arrow, Ctrl+V still work correctly