DST-101: feat: add Bayesian A/B & Thompson Sampling bandit, Retention LTV estimator, theme toggle, URL state sync, and portal index

[jasonchen36]

Summary of Enhancements

Extends the Decision Systems Toolkit with new statistical decision tools, interactive features, theme support, and a root portal landing page while preserving zero-dependency client-side privacy:

1. New Tool: Bayesian A/B Test & Multi-Armed Bandit (panel-bayes)

   • Calculates $P(B &gt; A)$ (Probability Test B outperforms Control A) using Beta-Binomial Monte Carlo sampling (20,000 draws in <10ms).
   • Computes expected decision risk/loss of selecting B over A.
   • Recommends Thompson Sampling multi-armed bandit traffic allocation split (e.g. 95% B / 5% A).

2. New Tool: Cohort Retention Decay & LTV Estimator (panel-retention)

   • Fits a Power-Law retention decay curve ($R(t) = a \cdot t^{-b}$) from Day 1, 7, 14, 30 cohort data.
   • Projects Day 60, Day 90, Day 180, and Day 365 retention rates and cumulative active user days.

3. URL State Hash Auto-Sync & Deep Linking

   • Serializes all input parameters into URL hash parameters (?tab=...&bayesAConv=100...), allowing users to share direct links to calculation results.

4. One-Click "Copy Executive Summary" Button

   • Formats calculation results into copy-pasteable Markdown summaries for Slack, MS Teams, JIRA, or emails.

5. Dark Mode & Print Support

   • Theme toggle button (☀️ / 🌙) with prefers-color-scheme auto-detection and localStorage persistence.
   • Clean @media print styling for PDF generation.

6. Root Portal Landing Page (index.html) & CI Workflow (.github/workflows/ci.yml)

   • Portal page with cards linking to English and Chinese versions.
   • Added GitHub Actions HTML/JS validation workflow.

Verification

• Validated HTML/JS across all 3 pages (index.html, index-en.html, index-cn.html).
• Verified Monte Carlo draws and Power-Law decay curve calculations in Node/JS.

[jasonchen36]

Hi @Grace-RGe! Whenever you have a chance, I'd love your review on this PR bringing Bayesian A/B testing, Thompson Sampling, LTV retention forecasting, dark mode support, and URL state sync to the toolkit. Thank you!