Add measurement tools (distance, HU probe, ROI) to the viewer

[aperson30]

What

Adds measurement tools to the case viewer:

• Distance — click two points → length in mm
• HU at point — click a point → Hounsfield value there
• Rectangle ROI — drag a box → area + mean/max/min HU
• Clear — removes all measurements (leaves the crosshair intact)

Built on Cornerstone's standard LengthTool / ProbeTool / RectangleROITool, added to the existing tool group alongside the crosshair/pan/zoom tools.

How it works

• setActiveMeasurementTool() hands the primary mouse button to the selected measure tool and disables crosshair/pan while measuring; clicking the tool again (or the crosshair button) returns to navigation.
• clearMeasurements() removes only measurement annotations, so the crosshair is preserved.
• Annotations are recolored to cyan (resting) / white (selected) instead of Cornerstone's default yellow/green, which clashed with the colored organ masks. (Defaults are merged, not replaced, so the value-label fonts are preserved.)

Correctness

• Distances use the volume's mm spacing from the NIfTI affine (the same geometry that aligns the MPR planes + masks).
• HU readouts read the volume's scalar values, which are HU-calibrated — confirmed by the viewer's HU-defined window presets (Soft Tissue W400/C40, Bone W1800/C400, etc.) rendering correctly.
• Recommended reviewer sanity check: probe air ≈ −1000, muscle ≈ 30–60, bone ≈ +300–1000 HU.

Testing

• Typecheck (tsc -b) ✓
• Vitest 32/32 ✓ (smoke-test mock updated with new exports)
• Production build ✓
• Manually verified in the viewer: distance, HU probe, ROI stats render with values; tool switching and Clear behave correctly.