Fix email width for long content

[arjunkomath]

Summary

• constrain the shared email container to a responsive 600px maximum width
• force long unbroken content to wrap without resizing email tables
• cover both alert and invitation templates with rendered HTML regression tests

Verification

• pnpm exec oxfmt --check lib/email/templates/base.tsx tests/email-templates.test.tsx
• pnpm exec vitest run tests/email-templates.test.tsx
• ./node_modules/.bin/tsc --noEmit
• pnpm lint (0 errors; 7 existing unrelated warnings)

Manual verification

• Pending in the email client where the build-failure width issue was observed.

[github-actions]

React Doctor found no new issues. 🎉

_{Reviewed by React Doctor for commit aa4f68f.}