fix(tui): accelerate diff viewer scrolling

[jlongster]

Issue for this PR

Internal follow-up to the diff viewer work.

Type of change

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

What does this PR do?

Routes the diff viewer patch pane scrollbox through the shared getScrollAcceleration() handling already used by other TUI scroll areas. This makes mouse/trackpad scrolling in long diffs respect configured scroll acceleration and scroll speed.

How did you verify your code works?

• Ran bun run typecheck from packages/opencode.
• Ran bun test --timeout 30000 test/cli/tui/diff-viewer.test.tsx from packages/opencode.
• Ran bunx prettier --check src/cli/cmd/tui/feature-plugins/system/diff-viewer.tsx.
• Ran git diff --check.
• Started the TUI preview with bun run dev and manually verified diff viewer scrolling behavior.
• The pre-push hook also ran the workspace bun turbo typecheck successfully.

Screenshots / recordings

Not included; this changes scrolling behavior rather than static rendering.

Checklist

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