feat: Email Draft Preview before launching

[sahare77]

Description
Users are anxious about sending automated emails without seeing what the merged variables (like {{first_name}}) or AI-generated content looks like.

Steps to Reproduce

1. Build a campaign.
2. Click Launch.
3. Notice you never see a final preview of a real email before it sends.

Expected Behavior
Add a "Preview" button in the Campaign Builder that selects a random Lead from the selected list, replaces merge tags, calls the AI generator (if applicable), and displays the resulting subject/body in a modal.

Implementation Hints

// frontend/campaign-builder.js
async function previewEmail() {
    const response = await api.post(`/campaigns/${id}/preview/`, { step_id: 1 });
    document.getElementById('preview-modal-body').innerText = response.data.body;
}

[sahare77]

Hello @Kuldeeep18, I want to work on this issue. Please assign it to me under GSSoC 2026.

[aaniya22]

pls assign this to me under gssoc

[sahare77]

/assign under gssoc'26