Preserve CancerVerse case ids so CV cards load correctly

[aperson30]

Root cause

itemToId did raw.match(/\d+/) on the case id, which stripped the CV_ prefix from "CV_00000001" and returned 1 as a number. The card then rebuilt it as PanTS_${1.padStart(8)} → PanTS_00000001, hitting PanTS endpoints. So CancerVerse cases silently rendered as PanTS thumbnails or failed to load.

Fix

Introduce CaseId = number | string: PanTS stays a bare number, CancerVerse keeps its full prefixed string so it routes to the CV endpoints. Threaded through every place the id flows:

• helpers/search.ts — itemToId returns the full string for CV (startsWith("CV")), number otherwise; exported CaseId type.
• components/Preview.tsx — id: CaseId; caseIdStr branches on CV vs PanTS, so the thumbnail, viewer route, and (downstream) nifti/seg/report URLs all use the right prefix.
• helpers/prefetchVolume.ts — accepts CaseId (the hover prefetch now warms the correct CV url).
• routes/Homepage.tsx — grid state, ingestItems, and the save/compare features accept CaseId; the compare tray also accepts a typed CV_… id.
• helpers/savedCases.ts — SavedCase.id accepts string; loadSavedCases no longer filters out string ids.

Fully backward-compatible for PanTS (still numeric).

Testing

• tsc -b clean
• 103/103 frontend tests pass (added itemToId CV coverage)

Known follow-ups (out of scope — not the reported bug)

• Viewer for CV: /case/CV_… loads the CT (backend dispatch works) and seg returns {"masks_available": false}; the viewer shows CT-only. Doesn't crash (getPanTSId tolerates the string), but full CT-only viewer polish (hiding mask UI) is a separate task.
• Compare page for CV: CV cards can now be selected to compare, but ComparePage still parses ids as PanTS — comparing two CV cases won't render correctly yet. Worth a follow-up if CV comparison is wanted.