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/api/client.js

@@ -39,7 +39,11 @@ export const AnalyticsApi = {
   topSenders: (take = 20) => api.get(`/analytics/top-senders?take=${take}`).then((r) => r.data),
   volume: (days = 90) => api.get(`/analytics/volume?days=${days}`).then((r) => r.data),
   heatmap: () => api.get('/analytics/heatmap').then((r) => r.data),
-  attachments: () => api.get('/analytics/attachments').then((r) => r.data)
+  categoryHeatmap: () => api.get('/analytics/category-heatmap').then((r) => r.data),
+  attachments: () => api.get('/analytics/attachments').then((r) => r.data),
+  sidebarCounts: () => api.get('/analytics/sidebar-counts').then((r) => r.data),
+  // Large take to retrieve all senders for the volume-tiers page.
+  allSenders: () => api.get('/analytics/top-senders?take=5000').then((r) => r.data)
 };
 
 export const CleanupApi = {