Motivation

• Avoid a hard UI freeze when preparing very large diff datasets (e.g. 100k lines) by showing a visible loading state so users get feedback during the expensive preparation step.
• Prevent metric updates from being recorded while preparation is still running so performance numbers remain meaningful.

Description

• Move diff dataset generation out of a blocking useMemo into an effect-driven flow using useEffect + setTimeout(0) so the UI can render a loading state before heavy computation runs.
• Add isPreparing state and initialize diffData via useState to allow controlled updates and to keep the viewer mounted while new data is prepared.
• Render an accessible overlay spinner/message on top of the viewer during preparation and skip commit/total metric updates while isPreparing is true.
• Add overlay CSS (.viewer-loading--overlay) to position and style the loading state (semi-transparent background and blur).

Testing

• Ran pnpm --filter demo build in the environment, which failed due to missing/unalikeable dev workspace dependencies (e.g. react-router-dom and workspace package resolution), so a full production build could not be validated here.
• No automated unit tests were added or run as part of this change.

Codex Task