Add smooth meshing strength presets

Kromacut could replace the current Smooth Meshing on/off control with a strength selector.

Possible options:

• None
• Minimal
• Medium
• Aggressive

The goal is to let users choose how much smoothing they want based on the model. Some prints may only need a light cleanup around jagged edges, while others may benefit from stronger smoothing. Users should also be able to turn smoothing off entirely when they want sharper pixel-accurate geometry or faster generation.

Suggested behavior:

• None should behave like smooth meshing is disabled today.
• One preset should preserve the current Smooth Meshing behavior so existing users do not lose the familiar result.
• Stronger presets should be treated carefully and validated against preview, STL export, and 3MF export.
• The setting should be understandable as a print/model quality option, not just a visual preview effect.

Important constraint:
Smooth meshing strength must not introduce gaps, overlaps, non-manifold geometry, or broken shared boundaries between colors. This is especially important because Smooth Meshing affects exported geometry, not only the 3D preview.

Related: #40

Desired outcome:
Users can choose the amount of smoothing that fits their model, from no smoothing to a more aggressive softened edge style, while keeping generated models printable and reliable.

Start by locating the current Smooth Meshing control and the generation paths used for preview, STL export, and 3MF export. Define the four strength presets around the existing behavior, then validate that each setting produces printable geometry without gaps, overlaps, non-manifold geometry, or broken shared color boundaries.