Add `style=` argument to `Take Screenshot` to apply CSS during capture

@Snooz82 is already working on this.

Since Aug 6, 2026.

## Use case

Screenshots of real pages are polluted by volatile content: blinking carets, animated spinners, ads, timestamps, avatars. `Take Screenshot` already offers `mask=` for rectangular pink overlays and `disableAnimations=`, but many stabilization needs are not rectangular masking: hiding an element entirely (so layout ignores it), forcing a fixed font, neutralizing box-shadows, or making a dynamic region transparent. Today the only workaround is injecting a style tag before the screenshot and removing it afterwards — extra keywords, and it mutates the page under test.

Playwright's screenshot `style` option applies a stylesheet only *while the screenshot is taken*: the page is untouched before and after. This is the standard tool for producing stable images, especially as groundwork for the planned visual-regression keyword.

## Proposed keyword / arguments

New named-only argument on the existing keyword:

```
Take Screenshot ... style=None
```

- `style` — CSS text applied while making the screenshot. Elements matching `visibility: hidden` collapse from view; `display: none` removes them from layout. Works for both page and element (`selector=`) screenshots.

```robotframework
Take Screenshot baseline selector=id=dashboard
... style=.ad-banner, .live-ticker { visibility: hidden !important; }

# Combine with existing stabilizers
Take Screenshot fullPage=True disableAnimations=True
... style=* { caret-color: transparent !important; }
```

## Playwright API

- [page.screenshot `style` option](https://playwright.dev/docs/api/class-page#page-screenshot-option-style) (also on [locator.screenshot](https://playwright.dev/docs/api/class-locator#locator-screenshot-option-style)), available since Playwright 1.41.

## Implementation notes

- `protobuf/playwright.proto`: one new optional string field on the existing screenshot options message.
- `node/playwright-wrapper`: pass `style` through to the `page.screenshot`/`locator.screenshot` call in the existing screenshot handler.
- Python: new named-only parameter in `take_screenshot` (`Browser/keywords/browser_control.py`), forwarded via `_create_screenshot_options`; docs table entry; stub regen; atest asserting a styled element is absent from the captured image.

## Backwards compatibility

Additive named-only argument with default `None` (no stylesheet applied), preserving current behavior exactly.