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.Application/DTOs/SearchDtos.cs

@@ -16,4 +16,11 @@ public record SearchRequestDto(
     bool? HasAttachments,
     bool FuzzyMatch = false,
     int Page = 1,
-    int PageSize = 50);
+    int PageSize = 50,
+    // Folder / label filters
+    bool? IsInInbox = null,
+    bool? IsStarred = null,
+    bool? IsTrashed = null,
+    string? GmailLabel = null,   // e.g. "SENT", "DRAFT", "SPAM"
+    string? Category = null,     // EmailCategory name, e.g. "Finance"
+    long? MinSizeBytes = null);