feat(frontend): add custom 404 page for unknown routes

frontend/src/App.tsx

@@ -1,5 +1,5 @@
 import React, { useEffect, useState } from 'react'
-import { BrowserRouter as Router, Routes, Route, Navigate } from 'react-router-dom'
+import { BrowserRouter as Router, Routes, Route } from 'react-router-dom'
 import AppShell from './components/AppShell'
 import Dashboard from './pages/Dashboard'
 import Toolkit from './pages/Toolkit'
@@ -11,6 +11,7 @@ import Settings from './pages/Settings'
 import Scans from './pages/Scans'
 import TaskDetails from './pages/TaskDetails'
 import Workflows from './pages/Workflows'
+import NotFound from './pages/NotFound'
 import ApiKeySetupScreen from './components/ApiKeySetupScreen'
 import ErrorBoundary from './components/ErrorBoundary'
 
@@ -34,7 +35,7 @@ export function AppRoutes() {
       <Route path={routes.settings} element={<Settings />} />
       <Route path={routes.task} element={<TaskDetails />} />
 
-      <Route path="*" element={<Navigate to={routes.dashboard} replace />} />
+      <Route path="*" element={<NotFound />} />
     </Routes>
   )
 }

frontend/src/pages/NotFound.tsx

@@ -0,0 +1,89 @@
+import React, { useEffect } from 'react'
+import { Link } from 'react-router-dom'
+import { motion } from 'framer-motion'
+import { useTheme } from '../components/ThemeContext'
+import { routes } from '../routes'
+
+export default function NotFound() {
+  const { theme } = useTheme()
+  const isLight = theme === 'light'
+
+  useEffect(() => {
+    document.title = '404 - Page Not Found | SecuScan'
+  }, [])
+
+  return (
+    <div className={`min-h-[75vh] flex items-center justify-center p-6 ${isLight ? 'bg-zinc-50' : 'bg-charcoal-dark'}`}>
+      <motion.div
+        initial={{ opacity: 0, y: 20 }}
+        animate={{ opacity: 1, y: 0 }}
+        transition={{ duration: 0.5, ease: 'easeOut' }}
+        className={`w-full max-w-lg border-4 border-black p-8 shadow-[8px_8px_0px_0px_rgba(0,0,0,1)] relative overflow-hidden ${
+          isLight ? 'bg-zinc-100' : 'bg-charcoal'
+        }`}
+      >
+        {/* Reuse the existing scanline animation class from index.css */}
+        <div className="login-scanline" />
+
+        {/* Top border decoration */}
+        <div className="absolute top-0 inset-x-0 h-1 bg-rag-red" />
+
+        {/* Content */}
+        <div className="flex flex-col items-center text-center space-y-6 relative z-10">
+          {/* SecuScan Branding inside the 404 card */}
+          <div className="bg-rag-amber text-black px-3 py-0.5 text-[9px] font-black uppercase tracking-widest inline-block shadow-[2px_2px_0px_0px_rgba(0,0,0,1)] font-mono">
+            SecuScan // Security
+          </div>
+
+          {/* Warning Icon Badge */}
+          <div className={`w-16 h-16 flex items-center justify-center border-4 border-black shadow-[4px_4px_0px_0px_rgba(0,0,0,1)] ${
+            isLight ? 'bg-zinc-200' : 'bg-charcoal-dark'
+          }`}>
+            <span className="material-symbols-outlined text-4xl text-rag-red animate-pulse">
+              gpp_bad
+            </span>
+          </div>
+
+          {/* Error Code & Headings Hierarchy */}
+          <div className="space-y-3">
+            <h1 className={`text-7xl font-mono font-black tracking-tighter ${
+              isLight ? 'text-zinc-900' : 'text-silver-bright'
+            } animate-glitch`}>
+              404
+            </h1>
+
+            <h2 className={`text-2xl font-bold uppercase tracking-wide ${
+              isLight ? 'text-zinc-800' : 'text-silver-bright'
+            }`}>
+              Page Not Found
+            </h2>
+
+            <h3 className={`text-[10px] font-mono font-bold uppercase tracking-[0.2em] italic ${
+              isLight ? 'text-zinc-500' : 'text-silver/50'
+            }`}>
+              Perimeter Breach // Mismatch
+            </h3>
+          </div>
+
+          {/* Divider line */}
+          <div className="w-full h-1 bg-black/10" />
+
+          {/* Explanation Message */}
+          <p className={`text-xs font-mono uppercase tracking-widest leading-relaxed ${
+            isLight ? 'text-zinc-600' : 'text-silver/60'
+          }`}>
+            The requested page does not exist or has been relocated outside the mapped perimeter matrix. Verification failed. Access denied or target route is not configured.
+          </p>
+
+          {/* Action Button */}
+          <Link
+            to={routes.dashboard}
+            className="w-full sm:w-auto inline-block text-center bg-rag-blue hover:bg-rag-blue/90 text-black px-8 py-3 text-[10px] font-black uppercase tracking-[0.3em] border-4 border-black shadow-[4px_4px_0px_0px_rgba(0,0,0,1)] hover:shadow-none hover:translate-x-0.5 hover:translate-y-0.5 transition-all italic"
+          >
+            Return to Dashboard
+          </Link>
+        </div>
+      </motion.div>
+    </div>
+  )
+}

frontend/testing/unit/AppRoutes.test.tsx

@@ -1,6 +1,7 @@
 import { render, screen, waitFor } from '@testing-library/react'
 import { MemoryRouter, useLocation } from 'react-router-dom'
 import { AppRoutes } from '../../src/App'
+import { ThemeProvider } from '../../src/components/ThemeContext'
 
 vi.mock('../../src/api', () => ({
   getHealth: vi.fn().mockResolvedValue({ status: 'operational' }),
@@ -41,24 +42,29 @@ function PathProbe() {
 }
 
 describe('App route fallback', () => {
-  it('redirects unknown routes to dashboard', async () => {
+  it('renders NotFound page for unknown routes', async () => {
     render(
-      <MemoryRouter initialEntries={['/not-a-real-route']}>
-        <AppRoutes />
-        <PathProbe />
-      </MemoryRouter>,
+      <ThemeProvider>
+        <MemoryRouter initialEntries={['/not-a-real-route']}>
+          <AppRoutes />
+          <PathProbe />
+        </MemoryRouter>
+      </ThemeProvider>,
     )
 
     await waitFor(() => {
-      expect(screen.getByTestId('path-probe')).toHaveTextContent('/')
+      expect(screen.getByTestId('path-probe')).toHaveTextContent('/not-a-real-route')
     })
+    expect(screen.getByText(/Perimeter Breach/i)).toBeInTheDocument()
   })
 
   it('renders the loaded dashboard summary', async () => {
     render(
-      <MemoryRouter initialEntries={['/']}>
-        <AppRoutes />
-      </MemoryRouter>,
+      <ThemeProvider>
+        <MemoryRouter initialEntries={['/']}>
+          <AppRoutes />
+        </MemoryRouter>
+      </ThemeProvider>,
     )
 
     expect(await screen.findByText(/Total Findings/i)).toBeInTheDocument()
@@ -67,9 +73,11 @@ describe('App route fallback', () => {
 
   it('renders the findings workspace', async () => {
     render(
-      <MemoryRouter initialEntries={['/findings']}>
-        <AppRoutes />
-      </MemoryRouter>,
+      <ThemeProvider>
+        <MemoryRouter initialEntries={['/findings']}>
+          <AppRoutes />
+        </MemoryRouter>
+      </ThemeProvider>,
     )
 
     expect(await screen.findByRole('heading', { name: /Findings/i })).toBeInTheDocument()