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.Infrastructure/Configuration/Options.cs

@@ -19,8 +19,10 @@ public class GoogleOAuthOptions
 public class GmailSyncOptions
 {
     public const string SectionName = "GmailSync";
-    public int PageSize { get; set; } = 100;
-    public int MaxParallelism { get; set; } = 4;
+    /// <summary>Message-id list page size (Gmail max is 500).</summary>
+    public int PageSize { get; set; } = 500;
+    /// <summary>Concurrent Gmail message fetches during sync (kept well under quota).</summary>
+    public int MaxParallelism { get; set; } = 8;
     public int MaxRetries { get; set; } = 5;
     /// <summary>Base delay in ms for exponential backoff.</summary>
     public int BackoffBaseMs { get; set; } = 500;