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/tests/state.test.ts

@@ -0,0 +1,60 @@
+import { state, resetState } from '@/js/state';
+import { describe, it, expect, beforeEach } from 'vitest';
+
+describe('State Management', () => {
+
+  // Test the initial state on import
+  describe('Initial State', () => {
+    it('should have the correct initial values', () => {
+      expect(state.activeTool).toBeNull();
+      expect(state.files).toEqual([]); // Use toEqual for arrays/objects
+      expect(state.pdfDoc).toBeNull();
+      expect(state.pdfPages).toEqual([]);
+      expect(state.currentPdfUrl).toBeNull();
+    });
+  });
+
+  // Test the resetState function
+  describe('resetState function', () => {
+    
+    // Before each test in this block, we'll "dirty" the state
+    // to ensure the reset function is actually doing something.
+    beforeEach(() => {
+      // 1. Modify the state properties to non-default values
+      state.activeTool = 'merge';
+      state.files = [{ name: 'dummy.pdf', size: 1234 } as File];
+      state.pdfDoc = { numPages: 5 }; // Mock PDF document object
+      state.pdfPages = [{}, {}]; // Mock page objects
+      state.currentPdfUrl = 'blob:http://localhost/some-uuid';
+
+      // 2. Create the DOM element that the function interacts with
+      //    The setup.ts file will clean this up automatically after each test.
+      document.body.innerHTML = '<div id="tool-content">Some old tool content</div>';
+    });
+
+    it('should reset all state properties to their initial values', () => {
+      // Call the function to test
+      resetState();
+
+      // Assert that all properties are back to their default state
+      expect(state.activeTool).toBeNull();
+      expect(state.files).toEqual([]);
+      expect(state.pdfDoc).toBeNull();
+      expect(state.pdfPages).toEqual([]);
+      expect(state.currentPdfUrl).toBeNull();
+    });
+
+    it('should clear the innerHTML of the #tool-content element', () => {
+      const toolContentElement = document.getElementById('tool-content');
+
+      // Sanity check: ensure the content exists before resetting
+      expect(toolContentElement?.innerHTML).toBe('Some old tool content');
+
+      // Call the function to test
+      resetState();
+
+      // Assert that the element's content has been cleared
+      expect(toolContentElement?.innerHTML).toBe('');
+    });
+  });
+});