Add an edge border/frame tool for generated models

Kromacut could add a border/frame tool that follows the outer edge of the generated model and uses a chosen color or filament.

The goal is to let users add a clean physical outline around a print without needing to edit the source image manually. This could help prints feel more finished, make edges easier to see, or create a consistent frame around irregular shapes.

Useful options could include:

• Toggle border/frame on or off.
• Choose the border color or filament.
• Choose border thickness/width.
• Choose whether the border reaches the highest layer, a fixed height, or only a few top/bottom layers.
• Preview the border in the 2D/3D views before export.
• Export the border as part of the STL/3MF output.

The border should follow the actual model edge, including irregular shapes. It should also behave predictably when smooth/blended edges are enabled, so the result does not look jagged, detached, or inconsistent around softened areas.

This could potentially live as a tool near the 2D preview toolbar, similar to crop and other image-prep tools, or as a 3D/export option if that ends up feeling more natural.

A simple first version could support one border style with color, width, and height options, then expand later if needed.

Start by tracing the 2D/3D preview toolbar and the STL/3MF export entry points to determine where model-edge geometry and display options belong. Define a small first version with one border style, color, width, and height behavior, then verify that irregular and smooth/blended edges remain connected in preview and export.