fix(tui): interaction improvements to diff viewer

[jlongster]

Issue for this PR

N/A

Type of change

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

What does this PR do?

This improves interactions in the TUI diff viewer.

The main fix is that next and previous file navigation now uses the ordered patch file list instead of relying on viewport-derived state. This makes navigation more reliable when the file tree is focused, when single-patch mode is enabled, and when switching between single-patch and all-patches views.

It also keeps the selected patch and file tree selection synchronized, scrolls the selected patch after render when needed, and keeps selected files highlighted with the active accent color even when they are marked reviewed.

The file tree navigation logic was moved into testable utilities with coverage for ordering, movement, and selection behavior.

How did you verify your code works?

• Ran bun typecheck from packages/opencode
• Push hook ran bun turbo typecheck
• Manually tested the diff viewer with multiple added and modified files

Screenshots / recordings

Not included.

Checklist

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

[github-actions]

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[github-actions]

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