Upstream integration cycle: pull request row presentation, loading layout stability, and hover previews #700

Tracking issue for Batch 5 of upstream integration cycle up to HEAD (2efb8178d8).

Targeted upstream commits

1. 9f0c9f725d (#12545): fix(web): reuse current PR status in the sidebar
2. 6a699f0f2f (#12721): fix(web): stabilize pull request loading layout
3. 9334924700 (#12719): fix(web): pull request chips share the link hover preview
4. 411da2a735 (#12536): feat(web): use the linked pull request row layout on the pull requests page

Adversarial review invariants

• Respect Pylon's multi-PR tracking and quota reservation models
• Preserve Pylon branding and responsive layout authority
• Typechecks and unit tests with 0 errors