feat: add draft preview mode to content editor

[collinsezedike]

Summary

• src/app/api/preview/route.ts — new POST endpoint that accepts { content: string } and returns { html: string } without persisting anything; serves as the preview API
• src/app/editor/page.tsx — added a Preview/Back to Editor toggle button; in preview mode the draft is rendered as sanitized HTML using the existing sanitizeHtml (DOMPurify) utility; draft state is fully preserved in React state so switching modes never loses edits

Test plan

• Open the editor, type some content, click Preview — rendered HTML should appear
• Click Back to Editor — content should be exactly as left
• POST /api/preview with { "content": "<p>hello</p>" } returns { "html": "<p>hello</p>" }
• POST /api/preview with non-string content returns 400

Closes #210

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