Fix scroll restore for first timeline item

[nepenth]

Summary

Fixes a scroll restoration edge case in useVirtualPaginator where timeline item index 0 was treated as missing because the anchor check used a generic falsy test.

When the restore anchor is the first rendered item, anchorItem is 0. The previous condition returned undefined, so pagination could skip scroll restoration for that valid anchor.

Change

• Treat only non-number anchor indexes as missing.
• Preserve 0 as a valid restore anchor.

Validation

• npm ci
• npx eslint src/app/hooks/useVirtualPaginator.ts
• git diff --check

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