Add image segmentation support to GUI

[tejasstanley]

This PR extends the GUI structure to support image segmentation alongside the existing image detection flow.
Changes include:

• Adds task-specific image segmentation sidebar, dataset viewer, inference, and evaluator tabs.
• Adds Cityscapes support for the image segmentation GUI.
• Adds shared GUI utilities under tabs/tasks/utils.py.
• Moves browse helpers out of perceptionmetrics/utils/gui.py.
• Uses TorchImageSegmentationModel for segmentation inference and evaluation.
• Adds live evaluation progress callbacks for segmentation evaluation.