Files sharing should show a clearer message when share creation is rate limited

Steps to reproduce 1. Sign in as a normal user. 2. Open the sharing dialog for a file or folder. 3. Create public link shares repeatedly until the ShareAPIController::createShare user rate limit is reached. 4. Try to create another share, either on the same item or another item. Expected behavior The UI should explain that share creation is temporarily rate limited, for example: > Share creation is temporarily rate limited. Please wait a few minutes before creating more shares. Ideally it should also make clear that this is a per-user/time-window creation limit, not a per-file or per-folder share limit. Actual behavior The request returns HTTP 429 / Too Many Requests, but the Files sharing UI does not give enough context for admins or users to understand why share creation stopped. This can be mistaken for a per-file share limit or a broken sharing dialog. Technical context OCA\Files_Sharing\Controller\ShareAPIController::createShare is intentionally protected with: Whe…