improve calibration file importing

[BryonLewis]

Stereo import improvements (stereo-import-improvements)

• Auto-discovers calibration in the parent folder root when using subfolder layout (.json/.npz with cal or calibration in the name), on both desktop and web
• Extracts calibration UI into ImportMultiCamCalibration.vue, shown for all stereo import modes (multi-folder, subfolders, and keyword)
• Prefills last calibration on dialog open via getLastCalibration (desktop settings / web session)
• Improves subfolder import UX — per-camera re-pick, clearer video source labels, file-count chip only for image sequences
• Adds shared helpers in stereoParentFolder.ts with unit tests; desktop IPC for findParentFolderCalibrationFile; web stashCalibrationFile for discovered files
• Fixes desktop folder picker by omitting file-type filters when opening directories
• Fixes track settings menu z-index so it renders above the timeline bar (unrelated UI fix bundled in)