Preserve viewer viewport across resize transitions

[hugolytics]

Summary

• preserve the current pan/zoom state across resize, narrow/wide layout, panel collapse, and fullscreen transitions
• track the last known good viewport and restore it if a resize produces an invalid zoom state
• keep the fullscreen pop-out fallback for embeds that block fullscreen and bump the release to 0.1.2

Verification

• npm test
• npm run build
• npm run docs:prepare-assets
• browser audit of embedded narrow-to-wide resize on localhost