feat(react-email): className on Image/Section/Column and linked Image via href

Closes #461

Two gaps that forced custom overrides when building real emails:

Changes

• className on Image, Section and Column, so a <style> media query can target them (stack columns, full-width images on mobile).
• href on Image: when set, the image is wrapped in <Link>. Covers clickable logos and banners, which Link can't express since it only renders props.text.

New props are .nullish(), so existing specs and LLM output without them stay valid.

Tests

• 3 new tests: classes reach the HTML, href wraps the image in <a>, a plain Image is unchanged. The first two fail without the change.
• packages/react-email: 36/36 pass, tsc --noEmit clean.

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