Improve GitHub plugin layout on mobile

[ymichael]

Summary

• make the issue/PR list respond to its own container width, switching between title-first compact rows and a full table at 48rem
• keep the labeled Open/Closed selector in compact rows and show assignee avatars only when someone is assigned
• tighten the wide Assignee, Status, Updated, and action columns so the title gets substantially more room
• keep label chips and loading skeletons aligned with the same container-based layout rules

Screenshots

Both screenshots use a 390 × 844 viewport with cached issue data.

Before	After

Testing

• pnpm exec turbo run typecheck build --filter=bb-plugin-github
• pnpm exec turbo run test --filter=bb-plugin-github --force (6 tests passed)
• browser QA from 390 px through 1440 px, including the compact/table transition, selector menu behavior, and horizontal-overflow checks