Saving Project Settings clears screenplay page-count target

[smith-and-web]

Reproduction

1. Open a screenplay project with a configured target_page_count (for example, 120).
2. Open Project Settings, change the genre or pen name, and save.
3. Read the project again: target_page_count is now null.

Expected

Saving unrelated project metadata preserves the screenplay page-count target.

Evidence

On main at 54712ef, src/lib/components/ProjectSettingsDialog.svelte sends only author_pen_name, genre, description, and word_target in update_project_settings. src-tauri/src/commands/crud.rs unconditionally assigns project.target_page_count = settings.target_page_count, so the omitted optional field clears the persisted value.

Found while implementing #225. This behavior predates the writing-statistics feature.