fix: app_exit keybind ignored when using <leader> (including default <leader>q)

[razor-x]

Fixes #3851

What does this PR do?

Summary

• Add a global app_exit handler so leader key combos reach exit even when prompt focus is blurred, while still respecting the “only exit when prompt is empty” rule in packages/opencode/src/cli/cmd/tui/app.tsx:183.
  Why this fixes it
• q was being consumed while the prompt was blurred by leader mode, so the prompt-level app_exit handler never ran. The new global handler uses useKeyboard and usePromptRef to exit consistently.

How did you verify your code works?

Built and ran locally, verified q exits the app as expected.

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