Enable ESLint no-unused-vars rule (#4367)

# Description of Changes
Enable ESLint [no-unused-vars
rule](https://typescript-eslint.io/rules/no-unused-vars/)

frontend/src/tests/convert/ConvertE2E.spec.ts

@@ -17,7 +17,6 @@ import * as fs from 'fs';
 
 // Test configuration
 const BASE_URL = process.env.BASE_URL || 'http://localhost:5173';
-const BACKEND_URL = process.env.BACKEND_URL || 'http://localhost:8080';
 
 /**
  * Resolves test fixture paths dynamically based on current working directory.
@@ -266,7 +265,6 @@ async function testConversion(page: Page, conversion: ConversionEndpoint) {
 }
 
 // Discover conversions at module level before tests are defined
-let allConversions: ConversionEndpoint[] = [];
 let availableConversions: ConversionEndpoint[] = [];
 let unavailableConversions: ConversionEndpoint[] = [];
 
@@ -275,7 +273,6 @@ let unavailableConversions: ConversionEndpoint[] = [];
   try {
     availableConversions = await conversionDiscovery.getAvailableConversions();
     unavailableConversions = await conversionDiscovery.getUnavailableConversions();
-    allConversions = [...availableConversions, ...unavailableConversions];
   } catch (error) {
     console.error('Failed to discover conversions during module load:', error);
   }

frontend/src/tests/convert/ConvertIntegration.test.tsx

@@ -11,7 +11,7 @@
 
 import React from 'react';
 import { describe, test, expect, vi, beforeEach, afterEach, Mock } from 'vitest';
-import { renderHook, act, waitFor } from '@testing-library/react';
+import { renderHook, act } from '@testing-library/react';
 import { useConvertOperation } from '../../hooks/tools/convert/useConvertOperation';
 import { ConvertParameters } from '../../hooks/tools/convert/useConvertParameters';
 import { FileContextProvider } from '../../contexts/FileContext';
@@ -53,10 +53,6 @@ vi.mock('../../services/thumbnailGenerationService', () => ({
 }));
 
 // Create realistic test files
-const createTestFile = (name: string, content: string, type: string): File => {
-  return new File([content], name, { type });
-};
-
 const createPDFFile = (): StirlingFile => {
   const pdfContent = '%PDF-1.4\n1 0 obj\n<<\n/Type /Catalog\n/Pages 2 0 R\n>>\nendobj\ntrailer\n<<\n/Size 2\n/Root 1 0 R\n>>\nstartxref\n0\n%%EOF';
   return createTestStirlingFile('test.pdf', pdfContent, 'application/pdf');

frontend/src/tests/convert/ConvertSmartDetectionIntegration.test.tsx

@@ -15,7 +15,6 @@ import axios from 'axios';
 import { detectFileExtension } from '../../utils/fileUtils';
 import { FIT_OPTIONS } from '../../constants/convertConstants';
 import { createTestStirlingFile, createTestFilesWithId } from '../utils/testFileHelpers';
-import { StirlingFile } from '../../types/fileContext';
 
 // Mock axios
 vi.mock('axios');
@@ -507,7 +506,7 @@ describe('Convert Tool - Smart Detection Integration Tests', () => {
   describe('Real File Extension Detection', () => {
 
     test('should correctly detect various file extensions', async () => {
-      const { result } = renderHook(() => useConvertParameters(), {
+      renderHook(() => useConvertParameters(), {
         wrapper: TestWrapper
       });