Remove abandoned HTML Preview Tool files and fix ESLint warnings

- Removed unused imports (Key, Palette, QrCode) from Layout.js
- Deleted all HTML Preview Tool files from repository
- Removed HtmlPreviewTool import and route from App.js
- Build now completes successfully without ESLint warnings
- Ready for deployment in CI environment

src/components/Layout.js

@@ -1,6 +1,6 @@
 import React, { useState, useEffect, useRef } from 'react';
 import { Link, useLocation } from 'react-router-dom';
-import { Home, Hash, FileText, Key, Palette, QrCode, FileSpreadsheet, Wand2, GitCompare, Menu, X, Database, LinkIcon, Code2, ChevronDown } from 'lucide-react';
+import { Home, Hash, FileText, FileSpreadsheet, Wand2, GitCompare, Menu, X, Database, LinkIcon, Code2, ChevronDown } from 'lucide-react';
 import ThemeToggle from './ThemeToggle';
 
 const Layout = ({ children }) => {