feat: add "Open in OrcaSlicer" export action

[wenghaishi]

One-click hand-off from a generated model to OrcaSlicer via its orcaslicer://open?file=<url> deeplink, available in the Export dropdown.

let me know if this is alright or there might be some better place for this button.

What

• Backend — new GET /export/slicer/{fmt}/{token}/model.{fmt} route that converts the workspace GLB to STL on the fly: bakes scene-graph transforms, reorients Y-up → Z-up, and normalizes the longest edge to a sane print size. The URL is intentionally path-only and ends in model.stl (no query string), because OrcaSlicer derives the import filename/format from the URL's final path segment. Uses ancestry-based path containment (not string-prefix).
• Electron — a slicer:open IPC that fires the deeplink and reports success/failure so the UI can fall back when OrcaSlicer isn't installed.
• Frontend — an Open in OrcaSlicer item in the Export dropdown, shown only for sliceable workspace meshes, plus a pure, unit-tested deeplink builder.

Why STL + deeplink

OrcaSlicer registers orcaslicer://open?file=<url>, downloads the URL, and imports it — and it cannot import GLB. So we convert to STL (which OrcaSlicer auto-repairs) and serve it at a deeplink-shaped URL.

Testing

• api/tests/test_export_router.py — conversion, Y→Z reorientation, print-size normalization, and guards (unsupported format, extension mismatch, malformed token, path traversal, sibling-prefix escape, 404).
• src/areas/generate/orcaSlicerLink.test.ts — exact deeplink string, no query string, url-safe base64 round-trip.
• npm run lint clean; npm run test:node green. Manual E2E verified on macOS (imports upright, ~50 mm).

Notes / follow-ups

• STL-first by design; 3MF (units + colour) is a natural fast-follow.
• "Not installed" handling is a best-effort error toast; OS-level detection could be added later.

Closes #346