feat: folder view — clicking sidebar items shows filtered email list

- New FolderView page at /app/folder/:slug renders a paginated email table
- Sidebar links now route to their respective folder views
- Extended SearchRequestDto with IsInInbox, IsStarred, IsTrashed, GmailLabel, Category, MinSizeBytes
- SearchService applies the new filters; GmailLabel does a label-id lookup (SENT/DRAFT/SPAM)
- Frontend folderToRequest() maps every slug to the correct search payload
- Email rows show sender, subject+snippet, attachment icon, size, date; bold for unread
- Pagination controls for large folders

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

src/InboxIntel.Infrastructure/Search/SearchService.cs

@@ -1,6 +1,7 @@
 using InboxIntel.Application.Abstractions;
 using InboxIntel.Application.Common;
 using InboxIntel.Application.DTOs;
+using InboxIntel.Domain.Enums;
 using InboxIntel.Infrastructure.Persistence;
 using Microsoft.EntityFrameworkCore;
 
@@ -32,6 +33,27 @@ public class SearchService : ISearchService
             q = q.Where(e => e.IsUnread == unread);
         if (r.HasAttachments is { } att)
             q = q.Where(e => e.HasAttachments == att);
+        if (r.IsInInbox is { } inbox)
+            q = q.Where(e => e.IsInInbox == inbox);
+        if (r.IsStarred is { } starred)
+            q = q.Where(e => e.IsStarred == starred);
+        if (r.IsTrashed is { } trashed)
+            q = q.Where(e => e.IsTrashed == trashed);
+        if (r.MinSizeBytes is { } minSize)
+            q = q.Where(e => e.SizeEstimateBytes >= minSize);
+        if (!string.IsNullOrWhiteSpace(r.Category) && Enum.TryParse<EmailCategory>(r.Category, true, out var cat))
+            q = q.Where(e => e.Category == cat);
+        if (!string.IsNullOrWhiteSpace(r.GmailLabel))
+        {
+            var labelId = await _db.Labels
+                .Where(l => l.UserId == userId && l.GmailLabelId == r.GmailLabel.ToUpperInvariant())
+                .Select(l => (Guid?)l.Id)
+                .FirstOrDefaultAsync(ct);
+            if (labelId.HasValue)
+                q = q.Where(e => e.EmailLabels.Any(el => el.LabelId == labelId.Value));
+            else
+                return new PagedResult<EmailSummaryDto> { Items = [], Page = r.Page, PageSize = r.PageSize, TotalCount = 0 };
+        }
 
         if (!string.IsNullOrWhiteSpace(r.Query))
         {