Add width/height attributes to images to prevent CLS

**Problem:**
`createOptimizedPicture` generates `<img>` elements without width/height attributes, causing Cumulative Layout Shift (CLS) and poor Core Web Vitals scores.

**Constraint:**
Solution must NOT require authors to manually add dimension metadata to documents (per boilerplate authoring guidelines).

**Related Issue:** adobe/aem-boilerplate#169