feature: persist split/unified diff preference and/or support ?diff=split on PR views #4720

Currently DiffView defaults diffStyle to "unified" in component state, so refreshing or navigating between PR rounds/commits resets the diff view back to unified every time.

Additionally, while standalone commit pages (/[handle]/[repo]/commit/[ref]) read ?diff=split from the URL query string, PR diff pages (PullViewPage.svelte) do not wire ?diff=split into DiffView.

It would be helpful to:

1. Persist the selected diff style (unified vs split) as a preference across page loads (e.g. via a cookie or user setting).
2. Support the ?diff=split query parameter on PR diff routes as well.