fix: bound diff result height

[lauridskern]

Summary

Constrain large file diff result blocks so long patches scroll inside the activity card instead of stretching the chat view.

Context

Resolves #22. Large diff outputs could dominate the chat transcript and make surrounding conversation content hard to reach.

Changes

• Added a bounded diff body container with max-height: min(36rem, 60vh).
• Enabled contained scrolling for both rendered git patches and plain preformatted patch output.
• Kept existing patch rendering behavior and options unchanged inside the scroll container.

Testing

• git diff --check
• cargo check --manifest-path src-tauri/Cargo.toml

Note: TypeScript build checks could not be run in this environment because npx is not installed.

Links

• Related issue: Give diff UI a max height #22