[NcProgressBar] No way to display numeric percentage value (value is never rendered as text)

### Is your feature request related to a problem? Please describe. When using `NcProgressBar`, there is currently no way to display the numeric percentage value (e.g. `42%`) alongside the visual progress indicator. Relevant code reference: - `value` is used only for progress calculation and ARIA - No text output or slot exists to render the numeric value ### Describe the solution you'd like A minimal solution could be an optional prop (e.g. `showValue`) that renders the percentage as an overlay or label for the linear progress bar, without changing the default appearance for existing users. ### Additional context This issue is visible in nextcloud/tables and tracked there as well: https://github.com/nextcloud/tables/issues/1885