feat(test): add comprehensive test suite and testing infrastructure

- Add vitest configuration with coverage reporting
- Create test setup file with DOM mocks
- Add tests for state management, helpers, and tool configurations
- Update tsconfig and package.json for testing support
- Clean up unused comments and improve code style

src/js/state.ts

@@ -12,5 +12,6 @@ export function resetState() {
     state.files = [];
     state.pdfDoc = null;
     state.pdfPages = [];
+    state.currentPdfUrl = null;
     document.getElementById('tool-content').innerHTML = '';
 }