feat: folder view — clicking sidebar items shows filtered email list

- New FolderView page at /app/folder/:slug renders a paginated email table
- Sidebar links now route to their respective folder views
- Extended SearchRequestDto with IsInInbox, IsStarred, IsTrashed, GmailLabel, Category, MinSizeBytes
- SearchService applies the new filters; GmailLabel does a label-id lookup (SENT/DRAFT/SPAM)
- Frontend folderToRequest() maps every slug to the correct search payload
- Email rows show sender, subject+snippet, attachment icon, size, date; bold for unread
- Pagination controls for large folders

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

frontend/src/main.jsx

@@ -6,6 +6,7 @@ import Dashboard from './pages/Dashboard.jsx';
 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 Layout from './components/Layout.jsx';
 import './styles.css';
 
@@ -22,6 +23,7 @@ ReactDOM.createRoot(document.getElementById('root')).render(
           <Route path="senders" element={<Senders />} />
           <Route path="cleanup" element={<Cleanup />} />
           <Route path="unsubscribe" element={<Unsubscribe />} />
+          <Route path="folder/:slug" element={<FolderView />} />
         </Route>
 
         <Route path="*" element={<Navigate to="/" replace />} />