Add expandUnchanged setting to show full files in diffs

[spaansba]

In the project I work on, we keep files small and focused. That makes the full file context crucial for understanding the effect of a change, so I end up expanding every hunk by hand. A setting to auto expand all unchanged lines would make that a lot nicer.

This adds an opt-in expandUnchanged setting. Enable it with View > Diff > Expand Unchanged Lines or "expandUnchanged": true in settings. It defaults to false, so current behavior is unchanged.

How it works

• Passes through to the existing expandUnchanged option of the @pierre/diffs CodeView. Patch-only diffs load their full contents through the existing loadDiffFiles loader as they render, so nothing extra loads up front.
• Stays off in read-only views without a loader (e.g. shared walkthroughs), since the library can't expand a patch-only diff there.
• Wired like showWhitespace: config defaults, schema, electron/config.cjs normalization and a View > Diff menu checkbox.

Tests

Adds a ReviewCodeView test that the option reaches CodeView when contents can be loaded and stays off in read-only views. vp check and vp test pass.

AI assistance

This PR was written primarily with Claude Code.

[cpojer]

Not loving this too much but I'm happy to merge it.