Add custom segmentation classes

[harleensachdev]

What this adds

Builds on #57 (mask editing). Lets a user create a brand-new annotation class at runtime (name + colour), paint it with the brush, see it in the organ list / hover-identify / 3D view, and download or save it alongside the edited mask.

How it works

• createNewAnnotationClass (CornerstoneNifti2.tsx) picks the next free segment index, registers its colour in Cornerstone's LUT, and remembers its name for the session.
• The mask-edit panel gained a "New class…" form wired to that function.
• Since custom classes have no pre-generated 3D mesh, extractSegmentSurface runs marching cubes (vtk.js) client-side on the live labelmap, debounced so it doesn't rebuild on every brush stroke.
• Saving to the server or downloading now also includes a labels.json sidecar (name + colour per custom index) whenever custom classes exist. The download path zips the mask + sidecar together so the metadata isn't lost, and the save-to-server path only sends the sidecar when there's actually something in it.

Testing done

• Created 2 custom classes on a sample case, painted each, confirmed correct 2D colour, hover tooltip, and live 3D mesh.
• Saved to server and downloaded the zip; confirmed labels.json contains the right name/colour pairs.
• Confirmed no labels.json is sent/created when no custom classes exist.
• Verified existing static-organ editing/saving/3D flows are unaffected.
• npm run build passes with no TypeScript errors.