root: Add bordered option for layer-shell fullscreen windows

[gaoyia]

Add Root::borderless() so that layer-shell fullscreen surfaces (greeter, lock screen, screensaver, panel taskbar, etc.) can skip the Linux CSD window_border wrapper, which otherwise paints an unwanted border and shadow around an edge-to-edge surface.

When borderless is set, Root::render returns the inner content directly instead of wrapping it in window_border(). The default is false, so existing behavior is unchanged.

Description

On Linux, Root::render always wraps its content in window_border() to draw client-side decoration borders and shadows. For layer-shell fullscreen surfaces (greeters, lock screens, screensavers, panels, etc.) this wrapper is undesirable: such surfaces are edge-to-edge and should not have a border or drop shadow around them.

This PR adds an opt-in Root::borderless(bool) builder method backed by a borderless: bool field (default false). When borderless is true, Root::render returns the inner content element directly; otherwise it keeps wrapping the content in window_border().shadow_size(...) exactly as before. Both branches end with .into_any_element() so the return type is unified.

Because the default is false, all existing windows are unaffected.

Break Changes

None.

How to Test

• cargo build -p gpui-component
• On Linux with client-side decorations, set Root::borderless(true) on a layer-shell fullscreen surface and confirm the border and shadow are no longer drawn around it.
• Confirm that a default Root (without borderless, or borderless(false)) still renders the usual border and shadow.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific)