feat(player): add native subtitle selector

[MagiqueDeveloper]

Summary

• Add a captions button beside the player transport controls.
• Use a native SwiftUI Menu for subtitle selection.
• Group options as None, Built-In, and External Subtitles.
• Reuse the existing subtitle-selection actions and shared option construction so the full settings panel remains consistent.

Verification

• xcodebuild ... build — passed.
• xcodebuild ... test -quiet — passed with exit code 0.
• Installed and launched the app on the tvOS simulator.
• git diff --check — passed.
• Independent staged-diff review — passed with no security or logic findings.

Imagery

Closes #35