Update modal background opacity to 50% for improved visibility

[mrleemurray]

Enhance the visibility of modal backgrounds by updating their opacity to 50%. This change applies to various modal components across the application, ensuring a consistent user experience. Test the modals to confirm the improved visibility.

Addresses: #317691

[Copilot]

Pull request overview

Updates modal backdrop dimming to a consistent 50% black across key modal surfaces, aligning workbench and Sessions UI behavior with the goal in #317691.

Changes:

• Increase modal editor backdrop opacity from 0.3 → 0.5.
• Increase base dialog modal dimming opacity from 0.3 → 0.5.
• Align Electron window-controls “dimmed” color blending to the 50% black overlay, and update associated inline documentation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/vs/workbench/browser/parts/editor/media/modalEditorPart.css	Raises modal editor background dimming to 50% black.
src/vs/sessions/contrib/chat/browser/media/runScriptAction.css	Raises Sessions “run script” modal backdrop dimming to 50% black.
src/vs/platform/windows/electron-main/windowImpl.ts	Updates window-controls dimming blend factor and comment to match 50% modal overlay.
src/vs/base/browser/ui/dialog/dialog.css	Raises dialog modal dimming to 50% black.