fix(datagrid): restore persisted column widths

[LarryHu0217]

Description

On the first Datagrid restore, saved column widths could be discarded in two places: the viewer schema filtered out the plugin-owned columns key, and the width restore could run before the plugin and regular-table had finished their initial draw.

This change:

• declares columns as passthrough plugin config so it survives schema filtering
• caches width overrides received before Datagrid initialization
• reapplies saved widths after the initial regular-table draw completes
• adds a regression test that verifies both the rendered width override and the subsequently saved config

Fixes #3163.

Validation

• PACKAGE=viewer-datagrid pnpm run lint
• pnpm --filter @perspective-dev/metadata run build
• pnpm --filter @perspective-dev/server run build
• pnpm --filter @perspective-dev/client run build
• pnpm --filter @perspective-dev/viewer run build
• pnpm --filter @perspective-dev/viewer-datagrid run build
• PACKAGE=viewer-datagrid pnpm exec playwright test --config=tools/test/playwright.config.ts packages/viewer-datagrid/test/js/column_settings.spec.ts --grep=First (1 passed)
• git diff --check

Documentation is not applicable because this fixes existing persistence behavior and does not add a public API.

AI assistance

I used OpenAI Codex to inspect the codebase, assist with the implementation and regression test, and run and review the validation steps. I reviewed the final diff and test results myself.