feat: Add PR summary header to diff viewer

[Dimillian]

Motivation

• Improve the PR review experience by showing the selected GitHub PR's title, metadata and rendered markdown description at the top of the diff viewer so users can see the PR context without leaving the diff.

Description

• Include body in the GitHub PR payload by requesting body from gh pr list in src-tauri/src/git.rs and adding body to the Rust and JS PR types (src-tauri/src/types.rs and src/types.ts).
• Pass the selected PR through the layout into the diff viewer by exposing selectedPullRequest in the layout options and wiring it through src/App.tsx and src/features/layout/hooks/useLayoutNodes.tsx.
• Render a PR summary section inside the diff viewer showing PR number, title, author, updated time, branches and the markdown-rendered body using the existing Markdown component in src/features/git/components/GitDiffViewer.tsx.
• Add styling for the PR summary card in src/styles/diff-viewer.css to present the header similarly to GitHub.

Testing

• Ran npm run lint, which completed with a TypeScript version warning but returned successfully.
• Ran npm run typecheck (tsc --noEmit) and it passed with no type errors.
• Started the dev server with npm run dev and captured a screenshot via a Playwright script to verify the UI render, which completed successfully.

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Codex Task