Add configurable width for side menu and adjacent content (subwindow)

[max-anders]

This update introduces a customContentWidth parameter to improve control over the width of the side menu and its adjacent content, specifically for handling the transition between collapsed and expanded states.

By default, the width is based on MediaQuery.of(context).size.width, but it can be customized for different layouts, particularly when using LayoutBuilder. This ensures a smoother and more predictable transition between states.

🔹 Changes & Improvements

Added customContentWidth to define the width for both the side menu and adjacent content.
Helps resolve issues where the transition between collapsed and expanded states was inconsistent due to full-screen width calculations.
Defaults to MediaQuery but allows custom values to fit within constrained layouts (e.g., when placed inside a Row).
Works seamlessly with LayoutBuilder.maxWidth for better responsiveness.

📌 Usage Example

When using the widget normally:

SideMenu(customContentWidth: MediaQuery.of(context).size.width * 0.7),

For better adaptability inside a constrained layout:

LayoutBuilder(
  builder: (context, constraints) {
    return SideMenu(customContentWidth: constraints.maxWidth);
  },
);

This ensures that the side menu and its adjacent content correctly adjust within flexible layouts rather than relying on the full screen width.

[Jamalianpour]

Hi @max-anders, thanks for your PR.
Before I accept and merge it, add document and comments to your code.