feat(editor): expose printable STL export

[nicemit]

What does this PR do?

Exposes printable STL export in the settings panel Export section. Users can now download printable STL files (as a zip) via a dedicated button, using the same safe print profile as the existing 3MF export (printScale: 100, printScope: levels, printContent: structure, printBase: none).

Follow-up to Discussion #331 — exposes the existing internal print-stl export path in the Settings UI.

How to test

1. Run bun dev and open http://localhost:3002
2. Open or create a project with printable geometry
3. Open the sidebar → Export section
4. Confirm both Export 3D print 3MF and Export 3D print STL files buttons are visible
5. Click Export 3D print STL files — a zip should download
6. Click Export 3D print 3MF — still works as before
7. Run bun --cwd packages/editor run test -- print-export-button

Screenshots / screen recording

Screenshot shows the new "Export 3D print STL files" button alongside the existing 3MF export in the Export panel.

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

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Note

Low Risk
UI-only wiring to an existing print-stl export path with shared preflight and options; no auth or data-model changes.

Overview
Adds a dedicated Export 3D print STL files control next to the existing 3MF export in the settings Export section, replacing the single generic print export button.

preparePrintExport now takes an optional format (print-3mf or print-stl, default unchanged) and passes it through to modelExport while keeping the same fixed print options (levels, structure-only, no base, 100% scale). A unit test asserts STL uses that same profile and returns zip artifacts with STL metadata.

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