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

app/plugins/builtins/mail/routes.py

@@ -587,6 +587,50 @@ async def delete_folder(
     await db.delete(folder)
 
 
+# ─── Empty Folder (soft-delete all mails in folder) ───
+
+
+@router.post("/folders/{folder_id}/empty")
+async def empty_folder(
+    folder_id: str,
+    db: AsyncSession = Depends(get_db),
+    current_user: dict = Depends(require_permission("mail:delete")),
+):
+    """Soft-delete all mails in a folder (sets deleted_at = now()).
+
+    The mails remain in the database but are hidden from the normal list.
+    Returns the number of mails that were emptied.
+    """
+    tenant_id = uuid.UUID(current_user["tenant_id"])
+    user_id = uuid.UUID(current_user["user_id"])
+    f_id = _parse_uuid(folder_id, "folder_id")
+    folder = (
+        await db.execute(
+            select(MailFolder).where(and_(MailFolder.id == f_id, MailFolder.tenant_id == tenant_id))
+        )
+    ).scalar_one_or_none()
+    if not folder:
+        raise HTTPException(404, detail={"detail": "Folder not found", "code": "not_found"})
+    account = await _get_account(db, folder.account_id, tenant_id, user_id)
+    await _check_delegate_access(db, account, user_id, "delete")
+    # Soft-delete all non-deleted mails in this folder
+    result = await db.execute(
+        select(Mail).where(
+            and_(
+                Mail.folder_id == f_id,
+                Mail.tenant_id == tenant_id,
+                Mail.deleted_at.is_(None),
+            )
+        )
+    )
+    mails = result.scalars().all()
+    now = datetime.now(UTC)
+    for mail in mails:
+        mail.deleted_at = now
+    await db.flush()
+    return {"emptied_count": len(mails)}
+
+
 # ─── Attachment Upload (F-MAIL-04) ───