Use native FlexLayout preferred panel sizes

[altendky]

The viewer currently corrects FlexLayout weights after panels disappear and intercepts window resizing to keep auxiliary panel sizes. Replace that workaround with native preferred sizes from caplin/FlexLayout#530, pinned to revision 6b947d0261d95800a9d1c10378bac3639c1b0d3c.

Auxiliary panels retain their chosen sizes while Model absorbs available space changes. Preserve saved placements, migrate older weight-only sizes before omitting workflow panels, and keep Model's branch flexible when tabs move or merge. Build the branch's source with the existing Vite toolchain so CI's install with scripts disabled remains supported.

Validation:

• All 48 workspace, Edit, and toolbar browser tests passed, including new legacy-width migration and Model-merge regressions.
• All 22 frontend test files passed, including native preference and nested-layout coverage.
• A clean npm install with scripts disabled, frontend build, changed-file repository hooks, and whitespace checks passed.