Owen/control docs p4

[OwenMcDonnell]

Description

Add snackbar, shadermask, selectionarea, and segmentedbutton example tests and images.

Summary by Sourcery

Provide example tests and sample images for SnackBar, ShaderMask, SelectionArea, and SegmentedButton controls; refine example scripts and update documentation to reference new test-generated images.

New Features:

• Add integration tests with screenshot assertions for SnackBar, ShaderMask, SelectionArea, and SegmentedButton examples

Enhancements:

• Refine SegmentedButton example labels, default selections, and selected icons
• Add keys to Text controls in SelectionArea example and wrap non-selectable text similarly
• Standardize example entry points using if name == "main" guard
• Update ShaderMask examples with consistent image sources, dimensions, and added inline documentation snippet
• Wrap NavigationRail example in SelectionArea to demonstrate selectable content

Documentation:

• Update control documentation to point to test-generated images under golden directories and include image captions

Tests:

• Adjust existing Slider integration test to target a single Slider instance
• Introduce new integration tests under flet.integration_tests for SnackBar, ShaderMask, SegmentedButton, and SelectionArea

[sourcery-ai]

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View logs

[FeodorFitsner]

Example image is broken: https://5b4f7d06.flet-docs.pages.dev/controls/selectionarea/?h=selection#basic-example
Example screenshot is missing: https://5b4f7d06.flet-docs.pages.dev/controls/shadermask/?h=shader#applying-linear-and-radial-gradientsshaders
Example code is centered: https://5b4f7d06.flet-docs.pages.dev/controls/snackbar/#counter