feat(web): add branch changes and review actions

[enisze]

What Changed

• Added Changes and Review actions beside the Git controls in chat headers.
• Added a configurable review prompt under Settings → Source Control → Text generation.
• Added branch diff navigation and review-prompt draft insertion without overwriting existing composer content.
• Added settings contracts, validation, tests, and source-control documentation.

Why

Reviewers can now inspect branch changes and start a focused review directly from the active thread. The configurable prompt supports team-specific review guidance while preserving existing draft context.

UI Changes

The chat header now includes Changes and Review buttons. The Changes action opens the branch diff, while Review inserts the configured prompt into the composer.

Before/after screenshots are not included.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes