fix: prevent flash of white theme on VS Code startup

[Mohamed998788]

Problem:
When opening VS Code, a brief white/light theme flash (FOUC - Flash of Unstyled Content) occurs during the splash screen loading phase. This is especially jarring for users with dark theme preferences, as they see a bright white screen for a second before the workbench loads the correct theme.

Cause:
The initial shell colors in the renderer process were defaulting to white because the stored user theme data (background/foreground) wasn't available early enough in the configuration or was being ignored if auto-detection was disabled.

Solution:
Synchronized the stored theme preference from the Main process to the Renderer process during window initialization.

Changes:

themeMainServiceImpl.ts: Added getStoredThemeData() to retrieve the last used theme's colors from the state service.
window.ts: Extended INativeWindowConfiguration to include storedThemeData.
windowImpl.ts: Populated the window configuration with the stored theme colors before launching the renderer.
workbench.ts: Updated the splash screen logic to use the storedThemeData as a reliable fallback, ensuring the correct background color is applied to the body immediately.
Result:
The splash screen now correctly matches the user's saved theme (Dark, Light, or High Contrast) from the very first millisecond of startup.