[Bug]: timeline lane hints advertise the default shortcut letter after a shortcut is rebound #966

Describe the bug

The empty-lane hints on the timeline hard-code the default shortcut letters. After a shortcut is rebound, the hint keeps advertising the old key, which no longer does anything.

Steps to reproduce

1. Keyboard Shortcuts: bind Add zoom to X (here shortcuts.json already had "addZoom": {"key": "x"}).
2. Open a project with no zoom region.

Expected

The zoom lane says "Press X to add zoom", like the toolbar button's tooltip ("Add a zoom at the current time X").

Actual

The lane says "Press Z to add zoom" and Z does nothing; X adds the zoom. The hint strings in src/i18n/locales/*/timeline.json (pressZoom, and the speed/trim/camera/audio siblings) embed the letter instead of taking the bound key.

Environment

OpenScreen 2.0.0-rc.8, macOS 26.5. Not platform-specific.