feat: add scrollable and pinned leading/trailing support to NavigationRail

[ndonkoHenri]

Fix #1923

Test code

import flet as ft


def main(page: ft.Page):
    page.add(
        ft.NavigationRail(
            height=280,
            width=140,
            selected_index=0,
            label_type=ft.NavigationRailLabelType.ALL,
            min_width=100,
            min_extended_width=180,
            scrollable=True,
            pin_leading_to_top=True,
            pin_trailing_to_bottom=True,
            leading=ft.IconButton(icon=ft.Icons.MENU, tooltip="Leading"),
            trailing=ft.IconButton(icon=ft.Icons.SETTINGS, tooltip="Trailing"),
            destinations=[
                ft.NavigationRailDestination(
                    icon=ft.Icons.LOOKS_ONE,
                    label="One",
                ),
                ft.NavigationRailDestination(
                    icon=ft.Icons.LOOKS_TWO,
                    label="Two",
                ),
                ft.NavigationRailDestination(
                    icon=ft.Icons.LOOKS_3,
                    label="Three",
                ),
                ft.NavigationRailDestination(
                    icon=ft.Icons.LOOKS_4,
                    label="Four",
                ),
                ft.NavigationRailDestination(
                    icon=ft.Icons.LOOKS_5,
                    label="Five",
                ),
                ft.NavigationRailDestination(
                    icon=ft.Icons.LOOKS_6,
                    label="Six",
                ),
            ],
        )
    )


ft.run(main)

Summary by Sourcery

Add configurability for scrollable navigation rail content and pinned leading/trailing controls, and cover the new behavior with an integration test.

New Features:

• Introduce pin_leading_to_top, pin_trailing_to_bottom, and scrollable properties on NavigationRail to control layout and scrolling behavior of leading, trailing, and destination items.

Enhancements:

• Document the new NavigationRail properties and provide an example usage in the Python API docstring.
• Wire new NavigationRail properties through the Flutter control to the underlying widget for runtime configuration.

Tests:

• Add an integration screenshot test validating scrollable NavigationRail with pinned leading and trailing controls.

[sourcery-ai]

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