# Fix table-v2 default flex columns in fixed row height mode

## Goal
Ensure `ListTableV2` keeps default columns flex-sized in fixed row height mode so columns expand to fill available width unless an explicit fixed width is configured.

## Requirements
- Preserve default flex sizing for non-fixed columns with no explicit `width`.
- Do not let fixed row height mode remove or bypass the flex sizing contract.
- Keep explicit column widths authoritative when a width is configured.
- Verify the section 8 demo case reflects the expected cell/header flex behavior.

## Acceptance Criteria
- [ ] In section 8 of the demo page, default columns expose flex sizing behavior and fill remaining width.
- [ ] Columns with explicit widths still use their fixed width behavior.
- [ ] Fixed row height mode does not change default column sizing semantics.

## Technical Notes
- Primary component is expected to be under `packages/base/data/list-table-v2.vue`.
- Check whether `el-table-v2` column construction and the cell/header CSS/render path diverge in fixed-height mode.
- Reuse existing column width logic rather than introducing a parallel sizing mechanism.