Shield: Add input sanitization and length validation to Contact form

Added `sanitizeInput` utility to escape HTML characters.
Updated `Contact.tsx` to sanitize inputs before sending via `emailjs`.
Added max length validation for Name (100), Subject (200), and Message (5000).
Updated tests to cover sanitization and validation logic, including adding `cleanup()` to prevent test leakage.

.jules/sentinel.md

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+## 2025-02-12 - Missing Test Cleanup
+**Vulnerability:** Tests in `src/pages/__tests__/Contact.test.tsx` were failing with duplicate elements because `cleanup()` was not being called between tests.
+**Learning:** `testing-library/react` usually handles cleanup automatically, but in this environment/setup, explicit `cleanup()` and `document.body.innerHTML = ''` in `afterEach` is required to prevent DOM state leakage.
+**Prevention:** Always include explicit `cleanup()` in `afterEach` blocks when writing component tests in this repository.