Warn before reusing a keyboard shortcut that's already taken

[jvanderen1]

Summary

• Recording a shortcut another action already uses shows a warning instead of silently stealing it. You can Unbind [the other action], Edit [the other action] Instead (same sheet), or Cancel.
• Override unbinds the previous owner so it stays empty (Unbound). The action you are editing keeps the new shortcut.
• While recording, menu key equivalents are paused so chords like ⌘T reach the editor. Conflict checks use the live binding table. ⌘. still uses the reserved Send Escape menu item.

Test plan

• Record a free shortcut — it saves immediately, no warning.
• Record a taken shortcut (for example ⌘T while New Tab owns it) — Shortcut Already in Use, not a new tab.
• Unbind [action] — the new action shows the shortcut; the old one is Unbound / No Shortcut.
• Bounce Edit [action] Instead a few times, then unbind — only the current action updates; the other stays unbound.
• Cancel leaves bindings unchanged.
• After recording, menu shortcuts (⌘T, etc.) work again.

Screencast

Update.Shortcuts-gh.mp4

[kitknox]

This doesn’t build as it seems to depend on changes in your other pending PRs. Either need to rebase directly on main or we need to get through the other PRs first. I had replied to the one in the queue a few days back I believe.

[jvanderen1]

I just fixed this PR. Will take a look at your comment on #433. Thanks!