Add full X/Y/Z calibration from standing-plate coupon scans

[jaak0b]

Extends the tool from XY-only to full X/Y/Z shrinkage and skew calibration. A flatbed scanner only reads a flat face, so the XZ and YZ planes come from the same lattice printed standing on-edge (thick slab, solid base, bicone funnel holes scannable from either face), then laid flat to scan. One parametric OpenSCAD model renders all three pre-oriented plates.

Highlights:

• Three downloadable plates (calibration_coupon_{xy,xz,yz}.stl); any subset works, the app auto-sorts uploads.
• Plane auto-identification: 1/2/3 solid diagonal ribs across the bottom-row cells next to the origin marker (XY=1, XZ=2, YZ=3). Additive dark geometry survives stringing, shadow and overextrusion, which killed the earlier drilled-dot code on rough on-edge prints. Unreadable codes refuse (never silently guessed as XY).
• Reader projects each cell diagonal through the fitted affine and classifies Otsu-binarised line profiles with a reject option (Chow's rule); profile geometry derives from the coupon spec (ribWidthMm, ringOuterDiameterMm), no tuned constants.
• Threshold polarity and card detection resolved by validating both hypotheses against known geometry (coupon grid / ISO card shape), fixing real scans with bright scanner-lid margins.
• Multi-plane combine reconciles X/Y/Z across plates; firmware output covers Klipper SET_SKEW XY/XZ/YZ, Marlin M852 I/J/K, RRF M556, plus per-axis scale.
• Scan page redesigned as a single flow: multi-file drop, duplicate/extra-scan flags, inline results, global firmware picker, reset-skew step.
• Tests: 104 unit tests over rendered-model fixtures, 7 Playwright e2e including real 35 MP scans of a printed diagonal-marked plate covering the full upload-to-correction flow.

Real-print validation status: XY plate verified on real scans; XZ/YZ physical prints in progress (skew sign to be cross-checked against the CaliLantern).