fix(deps): bump @shotstack/schemas to 1.13.0; handle optional asset src

[dazzatronus]

What

Bumps @shotstack/schemas 1.11.0 → 1.13.0 and adapts the SDK to its src-or-prompt change. (@shotstack/shotstack-canvas is already at the latest, 2.9.0.)

Why

1.13.0 makes asset.src optional — media assets can now be defined by a prompt (AI generation) instead of a src. That turns asset.src into string | undefined, which broke 7 sites that assumed it was always present.

Changes

• audio-player, image-player, video-player: skip loading when an asset has no src (nothing to load until it's generated).
• media-thumbnail-renderer: mark the thumbnail failed and skip when there's no src.

Testing

npm run verify:ci passes — lint, typecheck, jest, build, and the package contract (bundle umd 2.92 MB, under the 3.0 MB limit).

Supersedes the Dependabot bump in #121, which failed CI on exactly these type errors.

[dazzatronus]

Superseded by #126, which includes these changes rebased onto main.