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>

frontend/src/api/client.js

@@ -62,6 +62,40 @@ export const LayoutApi = {
   save: (layout) => api.put('/widgetlayout', layout)
 };
 
+export const SearchApi = {
+  folder: (slug, page = 1, pageSize = 50) =>
+    api.post('/search', folderToRequest(slug, page, pageSize)).then((r) => r.data),
+};
+
+function folderToRequest(slug, page, pageSize) {
+  const base = { page, pageSize };
+  switch (slug) {
+    // ── Mailbox ──
+    case 'inbox':     return { ...base, isInInbox: true, isTrashed: false };
+    case 'allmail':   return { ...base };
+    case 'unread':    return { ...base, isUnread: true };
+    case 'starred':   return { ...base, isStarred: true };
+    case 'sent':      return { ...base, gmailLabel: 'SENT' };
+    case 'drafts':    return { ...base, gmailLabel: 'DRAFT' };
+    case 'archive':   return { ...base, isInInbox: false, isTrashed: false };
+    case 'spam':      return { ...base, gmailLabel: 'SPAM' };
+    case 'trash':     return { ...base, isTrashed: true };
+    // ── Special filters ──
+    case 'large':     return { ...base, minSizeBytes: 5_000_000 };
+    case 'old': {
+      const d = new Date(); d.setFullYear(d.getFullYear() - 1);
+      return { ...base, to: d.toISOString().slice(0, 10) };
+    }
+    // ── Smart folders ──
+    case 'automated':   return { ...base, category: 'Notification' };
+    case 'finance':     return { ...base, category: 'Finance' };
+    case 'social':      return { ...base, category: 'Social' };
+    case 'shopping':    return { ...base, category: 'Promotional' };
+    case 'noreply':     return { ...base, query: 'from:noreply' };
+    default:            return { ...base };
+  }
+}
+
 export const ExportApi = {
   reportUrl: (format) => `/api/v1/export/report?format=${format}`
 };