fix: throw a clear error when a media asset has no src

[dazzatronus]

What

Restore a clean tsc on main. The audio/image/video players and the timeline thumbnail renderer read asset.src directly, but the generated @shotstack/schemas types mark src as optional — so each use was a string | undefined type error (7 in total). A red type-check gate also masks any genuinely new errors.

How

Guard src at each point of use and throw an explicit … asset is missing a 'src' error — consistent with the existing invalid-source handling — which also narrows the type to string. No changes to generated schema types, no !/as assertions.

Behaviour

A media clip whose asset has no src now fails with a clear error instead of building an undefined… URL and failing obscurely. A thumbnail for such a clip falls back to the existing solid-colour state.

Verify

• tsc --noEmit and tsc --project tsconfig.test.json --noEmit both clean (pushed with the pre-push gate enabled, no bypass).
• Full unit suite green (1802).