feat: compact mail folder tree + right-click Ordner leeren

- MailFolderTree: compact styling matching AI assistant SessionList (py-1, text-sm, depth*12+8 padding, no space-y)
- Added right-click context menu on folders with "Ordner leeren" entry
- Backend: POST /mail/folders/{folder_id}/empty soft-deletes all mails in folder
- Frontend API: emptyFolder() function added to mail.ts
- Mail.tsx: onFolderEmptied callback reloads mails and folders

frontend/src/api/mail.ts

@@ -475,6 +475,10 @@ export function deleteFolder(folderId: string): Promise<void> {
   return apiDelete<void>(`/mail/folders/${folderId}`);
 }
 
+export function emptyFolder(folderId: string): Promise<{ emptied_count: number }> {
+  return apiPost<{ emptied_count: number }>(`/mail/folders/${folderId}/empty`, {});
+}
+
 // ─── Mails ──────────────────────────────────────────────────────────────────
 
 export function fetchMails(folderId: string, page: number, sortBy?: string, sortOrder?: string): Promise<MailListResult> {