fix(player): support autoplay for externally selected episodes

[wiktorekdev]

Summary

Adds a shared playEpisodeAtIndex action for external player integrations. It resolves an aired episode from its original metadata index and sends it through the existing download, stream-resolution, and autoplay pipeline.

PR type

• Reproducible bug fix
• UI glitch/bug fix
• Behavior bug/regression fix
• Small maintenance only, with no UI or behavior change
• Docs accuracy fix
• Translation/localization only
• Approved larger or directional change

Why

Desktop MPRIS Previous needs to start the resolved previous episode using the same autoplay policy as Next. The shared player previously had no action for an externally selected episode, so Desktop could only open the stream picker instead of applying the configured autoplay flow.

Issue or approval

Approved by a maintainer in #653: NuvioMedia/NuvioDesktop#653.

UI / behavior impact

• No UI change
• Behavior changed only to fix a documented bug/regression

Policy check

• I have read and understood CONTRIBUTING.md.
• This PR is small, focused, and limited to one problem.
• This PR is not cosmetic-only.
• Any UI change fixes a linked glitch/bug and includes visual proof, or this PR has no UI change.
• Any behavior change fixes a linked bug/regression or has explicit approval, or this PR has no behavior change.
• This PR does not bundle unrelated refactors, cleanups, formatting, or drive-by changes.
• This PR does not add dependencies, architecture changes, migrations, or product-direction changes without explicit approval.
• I listed the testing performed below.

Scope boundaries

No UI, player controls, MPRIS, dependency, or platform-specific changes are included. This PR only adds the shared autoplay action required by the approved Desktop MPRIS fix.

Testing

• ./gradlew :composeApp:testAndroidHostTest --tests com.nuvio.app.features.player.skip.PlayerNextEpisodeRulesTest --no-daemon
• Verified that valid original indices resolve aired episodes and that invalid or unaired selections are rejected.

Screenshots / Video

Not a UI change.

Breaking changes

None.

Linked issues

Approved Desktop MPRIS fix in #653: NuvioMedia/NuvioDesktop#653.