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.Api/Program.cs

@@ -71,6 +71,14 @@ builder.Services.AddAuthentication(options =>
         options.SaveTokens = true;
         foreach (var scope in google.Scopes) options.Scope.Add(scope);
         options.Events.OnCreatingTicket = GoogleAuthEvents.OnCreatingTicketAsync;
+        // Force the consent screen so Google ALWAYS returns a refresh token.
+        // Without this, Google omits the refresh token on re-authorisation,
+        // leaving offline Gmail sync with no usable credential.
+        options.Events.OnRedirectToAuthorizationEndpoint = context =>
+        {
+            context.Response.Redirect(context.RedirectUri + "&prompt=consent");
+            return Task.CompletedTask;
+        };
     });
 
 builder.Services.AddAuthorization();