feat: newsletter GDPR deletion, rate-limit UX, e2e lifecycle tests, and global ErrorBoundary

[Mystery-CLI]

Summary of Changes

This PR resolves four backlog tasks across the Newsletter and Resilience areas:

1. #1404 — GDPR Data Deletion Request Page

• Created /account/privacy/delete page (frontend/src/app/account/privacy/delete/page.tsx).
• Implemented double-confirmation modal requiring the user to explicitly type the confirmation phrase DELETE MY DATA PERMANENTLY.
• Added anti-autofill attributes (data-lpignore, data-1p-ignore, autoComplete="off", spellCheck={false}) to ensure deliberate manual confirmation.
• Integrated with api.newsletterGdprDelete with accessible alert handling for error/success states.

2. #1405 — Rate-Limit-Aware UX for Newsletter Subscribe

• Created frontend/src/components/NewsletterSignup.tsx and refactored LandingPage.tsx to use it.
• Added client-side rapid resubmission cooldown to prevent spamming before triggering server 429s.
• Implemented automatic countdown timer UX for HTTP 429 rate limit responses.
• Allows immediate retry when the user edits or fixes a typo in their email address.

3. #1406 — E2E Coverage for Newsletter Lifecycle & GDPR Flows

• Created frontend/e2e/newsletter.spec.ts covering the full end-to-end lifecycle:
  • subscribe → confirm → GDPR export → GDPR delete (modal & phrase confirmation) → unsubscribe.
  • Uses unique disposable test email addresses per run.
  • Guaranteed teardown/cleanup in afterEach even on assertion failure to prevent staging list pollution.
• Added e2e/newsletter.spec.ts to MOCKED_SPECS in frontend/playwright.config.ts.

4. #1407 — Recreate Global ErrorBoundary

• Enhanced frontend/src/components/ErrorBoundary.tsx with reload page action, report issue link/action, and resilient fallback UI.
• Expanded frontend/src/components/__tests__/LandingPage.error-boundary.test.tsx with unit tests for global error catching, fallback rendering, report action, and onError callback dispatch.

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Closes #1404
Closes #1405
Closes #1406
Closes #1407

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