[AbianS]

Summary

• Video: the export only contains the ranges the steps play, re-encoded with VBR H.264 (software OpenH264 first, then hardware, then VP9, then the original recording as a last resort). Each segment's last frame lasts until the segment ends.
• Photos: frames are extracted as lossless PNG and encoded once at export as AVIF (@jsquash/avif in a pool of workers). The exporter ships AVIF as is and still converts anything else to WebP. The editor CSP adds 'wasm-unsafe-eval' for the encoder.
• player-core: loads single-file .openshowcase bundles as a stream, and exports readStoredZip.
• Playground: a new extension page that plays an exported bundle with the real player. It opens from the export dialog ("Test it in the playground") or from the popup header.
• Changeset: minor (all packages are in the fixed group).

Test plan

• pnpm run ci passes (biome, build, types, tests, size)
• Export a demo: photos inside the bundle are .avif (.webp means the wasm CSP isn't working)
• "Test it in the playground" opens the downloaded bundle
• The popup's playground icon opens the playground