feat: P5 UX polish - 404 stranka, ErrorBoundary, focus trap, ARIA

- NotFound.jsx: 404 stranka misto redirectu na / (lazy-loaded)
- ErrorBoundary: CSS tridy misto inline stylu, DEV error stack, odkaz na Dashboard
- useFocusTrap hook: Tab/Shift+Tab cycling, auto-focus, restore focus on close
- ConfirmModal: focus trap integrovan
- admin-error-stack CSS pro DEV chybovy vypis

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/admin/AdminApp.jsx

@@ -1,5 +1,5 @@
 import { lazy, Suspense } from 'react'
-import { Routes, Route, Navigate } from 'react-router-dom'
+import { Routes, Route } from 'react-router-dom'
 import { AuthProvider } from './context/AuthContext'
 import { AlertProvider } from './context/AlertContext'
 import ErrorBoundary from './components/ErrorBoundary'
@@ -45,6 +45,7 @@ const Invoices = lazy(() => import('./pages/Invoices'))
 const InvoiceCreate = lazy(() => import('./pages/InvoiceCreate'))
 const InvoiceDetail = lazy(() => import('./pages/InvoiceDetail'))
 const Settings = lazy(() => import('./pages/Settings'))
+const NotFound = lazy(() => import('./pages/NotFound'))
 
 export default function AdminApp() {
   return (
@@ -86,7 +87,7 @@ export default function AdminApp() {
               <Route path="invoices/:id" element={<InvoiceDetail />} />
               <Route path="settings" element={<Settings />} />
             </Route>
-            <Route path="*" element={<Navigate to="/" replace />} />
+            <Route path="*" element={<NotFound />} />
           </Routes>
         </Suspense>
         </ErrorBoundary>

src/admin/admin.css

@@ -2419,6 +2419,25 @@ img {
   }
 }
 
+/* Error stack (DEV only) */
+.admin-error-stack {
+  max-width: 600px;
+  max-height: 200px;
+  overflow: auto;
+  padding: 0.75rem 1rem;
+  margin: 0;
+  border-radius: var(--border-radius-sm);
+  background: var(--bg-tertiary);
+  border: 1px solid var(--border-color);
+  color: var(--danger-color);
+  font-family: var(--font-mono);
+  font-size: 11px;
+  line-height: 1.5;
+  text-align: left;
+  white-space: pre-wrap;
+  word-break: break-word;
+}
+
 /* Keyboard shortcut badge */
 .admin-kbd {
   display: inline-block;

src/admin/components/ConfirmModal.jsx

@@ -1,6 +1,7 @@
 import { useEffect } from 'react'
 import { motion, AnimatePresence } from 'framer-motion'
 import useModalLock from '../hooks/useModalLock'
+import useFocusTrap from '../hooks/useFocusTrap'
 
 export default function ConfirmModal({
   isOpen,
@@ -14,6 +15,7 @@ export default function ConfirmModal({
   loading = false
 }) {
   useModalLock(isOpen)
+  const trapRef = useFocusTrap(isOpen)
 
   useEffect(() => {
     if (!isOpen) return
@@ -70,6 +72,7 @@ export default function ConfirmModal({
         >
           <div className="admin-modal-backdrop" onClick={onClose} />
           <motion.div
+            ref={trapRef}
             className="admin-modal admin-confirm-modal"
             role="alertdialog"
             aria-modal="true"

src/admin/components/ErrorBoundary.jsx

@@ -1,10 +1,10 @@
 import { Component } from 'react'
 
 export default class ErrorBoundary extends Component {
-  state = { hasError: false }
+  state = { hasError: false, error: null }
 
-  static getDerivedStateFromError() {
-    return { hasError: true }
+  static getDerivedStateFromError(error) {
+    return { hasError: true, error }
   }
 
   componentDidCatch(error, info) {
@@ -16,29 +16,32 @@ export default class ErrorBoundary extends Component {
   render() {
     if (this.state.hasError) {
       return (
-        <div style={{
-          minHeight: '50vh',
-          display: 'flex',
-          flexDirection: 'column',
-          alignItems: 'center',
-          justifyContent: 'center',
-          gap: '1rem',
-          color: 'var(--text-secondary, #888)'
-        }}>
-          <p>Něco se pokazilo při načítání stránky.</p>
-          <button
-            onClick={() => window.location.reload()}
-            style={{
-              padding: '0.5rem 1.5rem',
-              borderRadius: '8px',
-              border: '1px solid var(--border-color, #333)',
-              background: 'var(--bg-secondary, #1a1a1a)',
-              color: 'var(--text-primary, #fff)',
-              cursor: 'pointer'
-            }}
-          >
-            Načíst znovu
-          </button>
+        <div className="admin-empty-state" style={{ minHeight: '60vh', justifyContent: 'center' }}>
+          <div className="admin-empty-icon" style={{ width: 80, height: 80, marginBottom: '1.5rem' }}>
+            <svg width="36" height="36" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round">
+              <path d="M10.29 3.86L1.82 18a2 2 0 0 0 1.71 3h16.94a2 2 0 0 0 1.71-3L13.71 3.86a2 2 0 0 0-3.42 0z" />
+              <line x1="12" y1="9" x2="12" y2="13" />
+              <line x1="12" y1="17" x2="12.01" y2="17" />
+            </svg>
+          </div>
+          <p style={{ marginBottom: '0.5rem' }}>Něco se pokazilo při načítání stránky.</p>
+          {import.meta.env.DEV && this.state.error && (
+            <pre className="admin-error-stack">
+              {this.state.error.message}
+              {this.state.error.stack && `\n${this.state.error.stack}`}
+            </pre>
+          )}
+          <div style={{ display: 'flex', gap: '0.5rem', marginTop: '0.5rem' }}>
+            <a href="/" className="admin-btn admin-btn-secondary">
+              Zpět na Dashboard
+            </a>
+            <button
+              onClick={() => window.location.reload()}
+              className="admin-btn admin-btn-primary"
+            >
+              Načíst znovu
+            </button>
+          </div>
         </div>
       )
     }

src/admin/hooks/useFocusTrap.js

@@ -0,0 +1,53 @@
+import { useEffect, useRef } from 'react'
+
+const FOCUSABLE = 'a[href], button:not([disabled]), input:not([disabled]), select:not([disabled]), textarea:not([disabled]), [tabindex]:not([tabindex="-1"])'
+
+/**
+ * Focus trap pro modaly - drzi focus uvnitr prvku.
+ * Vraci ref ktery se pripoji na modal kontejner.
+ */
+export default function useFocusTrap(isOpen) {
+  const ref = useRef(null)
+
+  useEffect(() => {
+    if (!isOpen || !ref.current) { return }
+
+    const container = ref.current
+    const previouslyFocused = document.activeElement
+
+    // Focus prvni focusable prvek v modalu
+    const focusable = container.querySelectorAll(FOCUSABLE)
+    if (focusable.length > 0) {
+      focusable[0].focus()
+    }
+
+    const handleKeyDown = (e) => {
+      if (e.key !== 'Tab') { return }
+
+      const nodes = container.querySelectorAll(FOCUSABLE)
+      if (nodes.length === 0) { return }
+
+      const first = nodes[0]
+      const last = nodes[nodes.length - 1]
+
+      if (e.shiftKey && document.activeElement === first) {
+        e.preventDefault()
+        last.focus()
+      } else if (!e.shiftKey && document.activeElement === last) {
+        e.preventDefault()
+        first.focus()
+      }
+    }
+
+    document.addEventListener('keydown', handleKeyDown)
+
+    return () => {
+      document.removeEventListener('keydown', handleKeyDown)
+      if (previouslyFocused && typeof previouslyFocused.focus === 'function') {
+        previouslyFocused.focus()
+      }
+    }
+  }, [isOpen])
+
+  return ref
+}

src/admin/pages/NotFound.jsx

@@ -0,0 +1,30 @@
+import { Link } from 'react-router-dom'
+import { motion } from 'framer-motion'
+
+export default function NotFound() {
+  return (
+    <motion.div
+      className="admin-empty-state"
+      style={{ minHeight: '60vh', justifyContent: 'center' }}
+      initial={{ opacity: 0, y: 20 }}
+      animate={{ opacity: 1, y: 0 }}
+      transition={{ duration: 0.4 }}
+    >
+      <div className="admin-empty-icon" style={{ width: 80, height: 80, marginBottom: '1.5rem' }}>
+        <svg width="36" height="36" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round">
+          <circle cx="12" cy="12" r="10" />
+          <path d="M16 16s-1.5-2-4-2-4 2-4 2" />
+          <line x1="9" y1="9" x2="9.01" y2="9" />
+          <line x1="15" y1="9" x2="15.01" y2="9" />
+        </svg>
+      </div>
+      <h2 style={{ fontSize: '1.5rem', fontWeight: 600, marginBottom: '0.5rem', color: 'var(--text-primary)' }}>
+        404
+      </h2>
+      <p>Stránka nebyla nalezena.</p>
+      <Link to="/" className="admin-btn admin-btn-primary" style={{ marginTop: '0.5rem' }}>
+        Zpět na Dashboard
+      </Link>
+    </motion.div>
+  )
+}