feat(campaign): add campaign A/B testing tools for headlines and images

[devogechukwu]

Summary

This pull request implements comprehensive campaign A/B testing tools enabling creators to configure, preview, test, and optimize campaign headlines, hero images, and CTA button text to identify which variants drive higher conversion rates.

Acceptance Criteria Implemented

• Creator sets up variants in campaign editor:

  • Implemented CampaignABTestingEditor component integrated into:
    • CampaignEditModal under a dedicated "A/B Testing Tools" tab.
    • CampaignWizardSteps inside the initial DetailsStep during campaign creation.
  • Creators can define Variant A (Control) and Variant B (or additional test variants) specifying:
    • Campaign headline text
    • Hero image URL with live thumbnail and preview
    • CTA button text
  • Live visitor preview demonstrates the exact rendering of each variant.

• Traffic split 50/50 by default (configurable):

  • Initialized with an automatic 50/50 balanced split across variants.
  • Interactive percentage sliders and controls allow creators to customize traffic splits (e.g. 70/30, 80/20), validating that total traffic allocation equals 100%.

• Winner can be locked in manually or auto-selected by significance:

  • Manual winner locking: Creators can lock in any winning variant with a single click, instantly routing 100% of future visitors to the winner.
  • Auto-selection by statistical significance: Integrated two-proportion z-test calculation computing z-scores, p-values, and confidence levels ((p < 0.05) / 95% confidence threshold with a minimum sample size requirement).
  • When winner selection mode is set to significance, the system automatically identifies and locks in the winning variant upon reaching statistical significance.

Key Changes

• apps/web/src/types/campaign-ab-testing.ts: Type definitions for variants, traffic splits, statistical significance metrics, and test configuration.
• apps/web/src/services/campaign-ab-testing.service.ts: Core domain service handling variant routing, impression tracking, conversion tracking, winner locking, and two-proportion z-test calculations.
• apps/web/src/hooks/use-campaign-ab-testing.ts: Deterministic client hook for variant routing, impression recording, and conversion event tracking.
• apps/web/src/components/modules/campaign/ab-testing/: Creator UI with variant inputs, traffic split sliders, live preview cards, and statistical significance badges.

Closes #1221

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