feat: add subject line preview in campaign builder

[saurabhhhcodes]

What changed

• Added a live mobile inbox preview beneath the subject field in the campaign builder.
• The preview updates as the subject line changes and shows a compact lock-screen style card.
• Kept the behavior scoped to email steps so the editor stays lightweight.

Why

• Long subject lines can get truncated in mobile inbox previews.
• This helps contributors see the user-facing impact of the subject before sending.

Validation

• git diff --check
• node --check /tmp/campaign-builder-inline.js

Closes #398

[coderabbitai]

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