#863 [2026 Analytics] Add cohort retention views for account activity

Summary

Resolves #863 by adding Cohort Retention Views for Account Activity to the Analytics workspace in stellar-dev-dashboard.
Adds cohort retention views for Stellar account activity to the Analytics dashboard (#863). Accounts are grouped by their first-seen period (Day, Week, Month) and subsequent retention decay rates are tracked across subsequent time periods.
This PR provides cohort matrix heatmaps and retention curve visualizations that group accounts by their first-seen period (Day, Week, or Month) and analyze subsequent retention over time.

• Domain Library (src/lib/cohortRetention.ts): Pure TypeScript calculation engine for cohort matrices, summary statistics, period headers, activity filtering (Payments, Smart Contracts, DEX Trades), CSV/JSON exports, and discriminated union error handling (CohortRetentionResult).
• UI Component (src/components/dashboard/CohortRetentionView.tsx): Heatmap matrix table with HSL-tailored dark mode color gradients, Recharts line chart for average retention curve, granularity controls, view mode toggles (% vs #), and export buttons.
• Integration: Embedded in Analytics.tsx and registered route /cohortRetention in routes.ts.
• Documentation: Detailed guide in docs/features/COHORT_RETENTION_ANALYTICS.md and changelog entry in CHANGELOG.md.

Closes #863

Ran unit and component tests:
pnpm run test:unit -- src/lib/__tests__/cohortRetention.test.ts src/components/dashboard/__tests__/CohortRetentionView.test.tsx (20 passed)
2. React UI Component (src/components/dashboard/CohortRetentionView.tsx):

• Modern, high-aesthetic heat map matrix table with HSL-tailored dark mode color gradients.
• Granularity selector (Daily, Weekly, Monthly).
• View mode toggle (Percentage % vs Account Count #).
• Interactive Recharts LineChart for the aggregated average retention curve.
• Download buttons for CSV and JSON exports.
• Accessible tooltips, ARIA attributes, and error/empty state fallbacks.
• Primary flow: Verified daily, weekly, monthly cohort calculations, retention percentages, active account matrices, aggregated curve data points, activity filtering, and CSV/JSON export generation.
• Boundary cases: Verified empty activity list handling, single-account cohorts, 100% retention across all periods, 0% retention decay, date boundary parsing (UTC month/week boundaries), and minCohortSize filtering.
• Failure cases: Verified null / non-array activity inputs, invalid granularity strings ('year'), negative period parameters, malformed records with invalid dates/missing IDs, calculation error objects, and graceful fallback when URL.createObjectURL is unsupported in JSdom/SSR.

3. Analytics Integration & Routing:
   • Embedded CohortRetentionView in src/components/dashboard/Analytics.tsx.
   • Registered /cohortRetention route in src/routes/routes.ts.

Merge requirements

4. Automated Tests:
   • src/lib/__tests__/cohortRetention.test.ts (16 tests): Tests primary flows (weekly/daily/monthly cohorts), boundary cases (empty datasets, single activity, 0% & 100% retention, timestamp parsing), and failure paths (null inputs, invalid granularity, negative parameters, malformed records).
   • src/components/dashboard/__tests__/CohortRetentionView.test.tsx (4 tests): Component tests validating UI rendering, controls, view mode toggles, and export triggers.
     A PR is merged only when every box below is true. See
     Merge requirements for the full policy.
5. User & Developer Documentation:
   • docs/features/COHORT_RETENTION_ANALYTICS.md: Complete guide covering architecture, features, compatibility, security, and migration/integration code snippets.
   • CHANGELOG.md: Entry under ## [Unreleased].

• All required CI checks pass on the latest commit (not just an earlier push).
• No required checks are failing, pending, or skipped — re-run or fix them; do not ask for a merge while any are outstanding.
• The branch has no merge conflicts with the target branch (rebase or merge master if GitHub shows "This branch has conflicts").
• Tests were added or updated for the change (primary flow, a boundary case, and a failure case).
• Docs were updated where behaviour, configuration, or security posture changed.

Acceptance Criteria Verification

• Objective implemented with clear handling for invalid input, unsupported environments, and failure paths.
• Automated tests cover the primary flow, at least one boundary case, and at least one failure case.
• User-facing documentation and developer guidance updated with compatibility, security, and migration notes.
• All 20 new automated unit tests passing cleanly.