Keep the mobile submit tap ahead of keyboard dismissal

[amadad]

Summary

Prevent the default focus transfer on the submit button's pointerdown so the prompt editor keeps focus through a tap. The click still owns the commit.

Why

On iOS, focus transfer happens before click. Moving focus from the editor to the submit button starts keyboard dismissal, which resizes the app shell while the tap is still resolving — so the submit races a viewport resize and can be lost. Keeping the editor focused for the duration of the tap removes the race; genuine focus changes elsewhere still dismiss the keyboard normally.

Scope

The handler is deliberately narrow — it only calls preventDefault() when all of the following hold:

• the pointer is the primary button (event.button === 0)
• the editor exists and is not destroyed
• the editor is already focused

Keyboard activation is unaffected: Enter/Space on the button dispatch click without pointerdown, so they never reach this path.

Verification

• apps/app — PromptBoxInternal.test.tsx, 67/67 passing, including a new regression test asserting pointerdown is default-prevented, document.activeElement is still the editor, and onSubmit still fires once on the subsequent click
• pnpm exec turbo run typecheck --filter=@bb/app clean