Fresh draft gains an extra empty paragraph after first reload #8

A fresh browser with no saved draft ends up with an extra empty paragraph after the first reload.

Steps to reproduce

1. Open the app in a new browser profile (empty IndexedDB).
2. Type some text into the first paragraph and wait a moment.
3. Reload.

Expected

The draft has the single paragraph that was edited.

Actual

After the reload, the draft has an extra empty paragraph besides the edited one. On main before #3 it came out as ["", "INT. HOUXY\nSE"], and with #3 as ["INT. HOUXY\nShe waits.SE", ""], so this predates #3. A likely cause: LocalProject.open runs twice against the same empty IndexedDB store (e.g. a React StrictMode double effect), and each run calls createDocument, so the two blank documents merge.

Acceptance criteria

• Opening a fresh draft and reloading leaves exactly the paragraphs the user created.
• A regression test covers opening the same empty project twice concurrently.

Addresses #1 and #3