Add an on-demand file diff preview

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用户问题 / User problem

更改列表目前只显示路径与行数，用户在提交前查看具体改动时必须离开 RepoPuck。

The change list shows paths and line counts, but users must leave RepoPuck to review the exact change before committing.

Smallest useful scope

• Open a read-only diff panel when a text-file row is activated.
• Separate staged and unstaged content.
• Load on demand and enforce byte/line limits.
• Explain binary, oversized, deleted, and unavailable diffs.
• Do not add hunk staging in the first implementation.

Acceptance criteria

• Keyboard and pointer users can open and close the preview.
• The main list remains responsive with 1,000 changed files.
• Repository content is never persisted or sent to a network service.
• Temporary-repository tests cover staged, unstaged, binary, and truncated cases.