feat: add subject preview in campaign builder

[saurabhhhcodes]

Adds a mobile inbox-style subject preview to the campaign builder so users can see how their email subject and first-body lines will appear on a phone notification.

What changed:

• added a compact notification-style preview card under the subject input
• bound the preview to the live subject/body fields
• kept the preview synced when content changes through AI draft application or merge-tag insertion

Validation:

• git diff --check
• verified the new preview hooks and sync paths in the rendered builder markup

[coderabbitai]

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