Fix potential runtime error from unguarded image array access in PostSimple

[Copilot]

Accessing prev.images[0] and next.images[0] without optional chaining throws at runtime when a post's images field is undefined.

Changes

• layouts/PostSimple.js: Use optional chaining on images array access for both prev and next navigation cards

// Before
image={prev.images[0]}
image={next.images[0]}

// After
image={prev.images?.[0]}
image={next.images?.[0]}

When images is absent, the expression evaluates to undefined, and HorizontalCard already falls back to /static/images/default-ogp.png.

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