feat: rebuild Experiments tab with Difference-in-Differences analysis and Bayesian verdicts

Replace the basic CRUD experiment tracker with a scientifically rigorous A/B testing system:

- Add DiD analysis engine (services/experimentAnalysis.ts) that computes treatment vs control
  group comparisons across 7 metrics (units, sessions, CVR, CTR, ROAS, revenue, ACOS)
- Implement Bayesian verdict system (Winner/Loser/Inconclusive) using posterior probability
  with normal CDF approximation (Abramowitz & Stegun erf, no external deps)
- Build counterfactual time series for trend charts (actual vs estimated without change)
- Rewrite ExperimentsView as single component with 3 inline sub-views (list, detail, create)
  replacing the previous modal-based ExperimentDetail and ExperimentForm
- Add control group support, change annotations (before→after diffs), and SEO experiment type
- New types: ExperimentChangeAnnotation, DiDMetricResult, DifferenceInDifferencesResult,
  ExperimentVerdict
- Simplify App.tsx by removing experiment modal state (5 useState hooks eliminated)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

components/ExperimentBadge.tsx

@@ -98,11 +98,12 @@ export const ExperimentTypeBadge: React.FC<ExperimentTypeBadgeProps> = ({ type,
     lg: 'px-3 py-1.5 text-sm',
   };
 
-  const typeLabels = {
+  const typeLabels: Record<string, string> = {
     pricing: 'Pricing',
     advertising: 'Advertising',
     content: 'Content',
     promotion: 'Promotion',
+    seo: 'SEO',
   };
 
   return (