[migration] Import logo, navigation style, collections, captions

- Import site logo from legacy profile (`header_image`, `logo` field)

- Translate legacy nav styles: horizontal, vertical to sidebar, center to horizontal-centered, none to horizontal

- Import navigation colors and font settings

- Detect legacy filtered pages and create collection-enabled sections with tag filters

- Full caption import from `asset.title` and `i18n.[lang].title` (not just alt text)

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