refactor: Extract reusable UI components and add TDD documentation

- Add comprehensive TDD documentation to CLAUDE.md with coverage requirements and examples
- Extract reusable UI components to frontend/src/components/ui/ (Modal, FormInput, Button, Alert, etc.)
- Add shared constants (schedulePresets) and utility hooks (useCrudMutation, useFormValidation)
- Update frontend/CLAUDE.md with component documentation and usage examples
- Refactor CreateTaskModal to use shared components and constants
- Fix test assertions to be more robust and accurate across all test files

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/test/setup.ts

@@ -33,6 +33,9 @@ global.IntersectionObserver = vi.fn().mockImplementation(() => ({
 // Mock scrollTo
 window.scrollTo = vi.fn();
 
+// Mock scrollIntoView
+Element.prototype.scrollIntoView = vi.fn();
+
 // Mock localStorage with actual storage behavior
 const createLocalStorageMock = () => {
   let store: Record<string, string> = {};