feat: search bar in topbar with Gmail-style query syntax

- Search form in topbar routes to /app/search?q=...
- SearchResults page with infinite scroll, same row style as FolderView
- Supports Gmail-like operators: from:, is:unread, has:attachment, after:, before:
- SearchApi.query() calls GET /search which runs through the existing GmailQueryParser
- Input sanitised via encodeURIComponent on submit; external links use noopener/noreferrer

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

frontend/src/main.jsx

@@ -7,6 +7,7 @@ import Senders from './pages/Senders.jsx';
 import Cleanup from './pages/Cleanup.jsx';
 import Unsubscribe from './pages/Unsubscribe.jsx';
 import FolderView from './pages/FolderView.jsx';
+import SearchResults from './pages/SearchResults.jsx';
 import Layout from './components/Layout.jsx';
 import './styles.css';
 
@@ -24,6 +25,7 @@ ReactDOM.createRoot(document.getElementById('root')).render(
           <Route path="cleanup" element={<Cleanup />} />
           <Route path="unsubscribe" element={<Unsubscribe />} />
           <Route path="folder/:slug" element={<FolderView />} />
+          <Route path="search" element={<SearchResults />} />
         </Route>
 
         <Route path="*" element={<Navigate to="/" replace />} />