feat: Smart Folders sidebar, category heatmap, sidebar counts API

- Replace plain activity heatmap with CategoryHeatmapWidget on dashboard
- Add collapsible Smart Folders sidebar with Favorites, Mailbox, Smart Folders sections
- Favorites: customisable pinned shortcuts (default: Inbox, Unread, Large, Old, Read Later); pin/unpin smart folders via buttons; persists to localStorage
- Mailbox section: 11 Gmail mailbox items with icons and live count badges
- Smart Folders section: 10 category folders sorted by count desc
- Live count badges via new GET /api/v1/analytics/sidebar-counts endpoint
- Backend: SidebarCountsDto, GetSidebarCountsAsync with label lookups for SENT/DRAFT/SPAM, size/age filters, EmailCategory mapping
- Sidebar collapses to icon-only; section states persist to localStorage

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

frontend/src/main.jsx

@@ -3,6 +3,7 @@ import ReactDOM from 'react-dom/client';
 import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom';
 import Landing from './pages/Landing.jsx';
 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 Layout from './components/Layout.jsx';
@@ -18,6 +19,7 @@ ReactDOM.createRoot(document.getElementById('root')).render(
         {/* Authenticated app */}
         <Route path="/app" element={<Layout />}>
           <Route index element={<Dashboard />} />
+          <Route path="senders" element={<Senders />} />
           <Route path="cleanup" element={<Cleanup />} />
           <Route path="unsubscribe" element={<Unsubscribe />} />
         </Route>