**Task**: Fix list-table-v2 fixed column cropping and horizontal scroll
### Summary
Fixed list-table-v2 issues:
1. Fixed columns were cropped even with width:120 - el-table-v2 needed table-level fixed=true prop
2. Added hasFixedColumns computed to detect fixed columns and pass :fixed to el-table-v2
3. Fixed column width logic: fixed columns without explicit width now use minWidth as width WITHOUT flexGrow (flexGrow caused cropping in el-table-v2's internal fixed sub-table layout)
4. Non-fixed columns keep flexGrow:1 for auto-distribution
5. Horizontal scrolling for non-fixed tables with flexGrow is mutually exclusive - flexGrow fills space, preventing overflow