[migration] Fix fixed-width layout mapping and import all i18n versions

- Account for legacy `margin-right`, `padding` values when calculating column config

- Map legacy `fixedwidthconfig` gap/margin to new column layout gutter setting

- Preserve visual density (e.g. 3 items at 300px with 10px gap on 960px container = 3 columns with 10px gutter)

- Import ALL i18n language variants (currently only imports German preferentially)

- Set `default_language` from legacy user language field, `languages` array from found content

**Files:** `app/api/migration/import-stream/route.ts`, `src/lib/legacy-layout-mapper.ts`