importProjectFromJSON only checked a few top-level fields before writing
the project to storage, so a malformed .opencut.json could be saved and
only fail later when the project is opened. Add a Zod schema that validates
the full timeline shape (scenes -> tracks -> elements), settings, and
timeline view state before saveProject, replacing the presence checks and
the unchecked `as ExportedProjectJSON` cast. Zod is already a dependency,
so no new deps. Extra/unknown fields are tolerated for forward
compatibility; the raw parsed object is persisted so no element fields are
dropped.
30179bfdc3