feat(admin): build email preview, analytics, blockchain replay, and content management pages

[Grace-CODE-D]

Description

This PR resolves issues #1396, #1397, #1398, and #1399 by implementing four core Admin Dashboard pages and supporting design primitives:

1. Email Template Preview & Test-Send Page ([Admin Dashboard] Build email template preview and test-send page #1396)

   • Path: frontend/src/app/admin/email/preview/page.tsx
   • Consumes GET /api/v1/email/preview/:template_name and POST /api/v1/email/test.
   • Renders template HTML inside a sandboxed <iframe> (sandbox="" via srcDoc) to ensure isolation and prevent XSS risks while permitting external resources (images/fonts) to load safely.
   • Includes template selector, subject inspector, HTML/Text/Source view toggles, and live test-send dispatching.

2. Email Delivery Analytics Page ([Admin Dashboard] Build email analytics page #1397)

   • Path: frontend/src/app/admin/email/analytics/page.tsx
   • Consumes GET /api/v1/email/analytics to display open, click, delivery, and bounce rates.
   • Strictly guards against division-by-zero on zero-send periods or fresh deployments by safely returning 0% / N/A without NaN or Infinity.
   • Provides time window and template filtering along with summary KPI metric cards and daily breakdown data tables.

3. Blockchain Replay / Admin Tooling Page ([Admin Dashboard] Build blockchain replay/admin tooling page #1398)

   • Path: frontend/src/app/admin/blockchain/replay/page.tsx
   • Consumes POST /api/blockchain/replay for state-mutating event replays.
   • Uses the modal component with backdrop-dismiss disabled (disableBackdropDismiss={true}) and requires typing an explicit confirmation phrase (CONFIRM REPLAY).
   • Fully disables action inputs and controls for unauthorized admin sessions lacking the blockchain:replay permission tier.

4. Content Management Page ([Admin Dashboard] Build content management page #1399)

   • Path: frontend/src/app/admin/content/page.tsx
   • Consumes GET/POST /api/v1/content using form primitives (Form, FormField, Input, Textarea, Button).
   • Surfaces server field-level validation errors directly on invalid fields while keeping the admin's draft edits intact without discarding state on validation failure.

5. Shared Admin Primitives & Layout

   • frontend/src/app/admin/layout.tsx: Unified admin navigation and breadcrumbs (Create Component Library Documentation with Storybook #89).
   • frontend/src/components/admin/Modal.tsx: Accessible focus-trapping modal with backdrop-dismiss prevention (Market Cancellation & Global Refund Logic #16).
   • frontend/src/components/admin/Form.tsx: Reusable WCAG-compliant form primitives (Multi-Asset Support & Decimal Precision Handling #15).
   • frontend/src/styles/admin.css: Theme-aware styling matching PredictIQ design tokens.

---

Closes #1396
Closes #1397
Closes #1398
Closes #1399

[drips-wave]

@Grace-CODE-D Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits