fix: make session scrollbar track visible on default theme

[Kushalk0677]

The session scrollbar does not render in the TUI on the default opencode theme — the track background color blends into the surrounding panel making it invisible.

Issue for this PR

Closes #25981

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Changes the scrollbar track colors to match the sidebar's existing working approach: track backgroundColor uses theme.background (#0a0a0a) so it blends into the session viewport, and foregroundColor uses theme.borderActive (#606060) for a visible thumb. The sidebar uses the same pattern at sidebar.tsx:40-44.

How did you verify your code works?

• Ran bun dev . with scrollbar_visible temporarily set to true
• Verified the scrollbar thumb is visible against the dark session background
• Confirmed the track disappears naturally into the viewport background

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Based on the search results, I found related PRs addressing similar scrollbar visibility issues:

Related PRs:

1. PR fix: make scrollbar visible in Monokai theme #24507 - "fix: make scrollbar visible in Monokai theme"

   • Addresses scrollbar visibility in a different theme variant
   • Similar approach to fixing contrast/visibility issues

2. PR fix(tui): improve session scrollbar contrast #17446 - "fix(tui): improve session scrollbar contrast"

   • Older PR focused on scrollbar contrast improvements
   • Related to the same feature area

These PRs tackle similar scrollbar visibility problems but target different themes or are from earlier attempts. PR #26009 (the current PR) appears to be the focused fix for the default theme specifically, closing issue #25981.

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍