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>

src/InboxIntel.Application/Abstractions/IServices.cs

@@ -23,7 +23,9 @@ public interface IAnalyticsService
     Task<IReadOnlyList<SenderStatDto>> GetTopSendersAsync(Guid userId, int take = 20, CancellationToken ct = default);
     Task<IReadOnlyList<TimeSeriesPointDto>> GetVolumeOverTimeAsync(Guid userId, int days = 90, CancellationToken ct = default);
     Task<IReadOnlyList<HeatmapCellDto>> GetHeatmapAsync(Guid userId, CancellationToken ct = default);
+    Task<IReadOnlyList<CategoryHeatmapCellDto>> GetCategoryHeatmapAsync(Guid userId, CancellationToken ct = default);
     Task<IReadOnlyList<AttachmentBreakdownDto>> GetAttachmentBreakdownAsync(Guid userId, CancellationToken ct = default);
+    Task<SidebarCountsDto> GetSidebarCountsAsync(Guid userId, CancellationToken ct = default);
     Task RefreshAggregatesAsync(Guid userId, CancellationToken ct = default);
 }