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/pages/Mail.tsx

@@ -736,6 +736,7 @@ export function MailPage() {
               selectedFolderId={selectedFolderId}
               onSelect={handleSelectFolder}
               loading={loadingFolders}
+              onFolderEmptied={() => { loadMails(); loadAllFolders(); }}
             />
           </div>
         </ResizablePanel>