Moved subtitle selector into it's own file and unit test

[TheyCallMeSpy]

Pull Request

Summary

Extracts subtitle track selection logic from item_detail_screen.dart into a standalone subtitle_track_logic.dart utility, making it pure and unit-testable. Adds fixture-based unit tests and a fixture loader helper.

Type of Change

• Bug fix
• New feature
• Refactor
• Performance improvement
• UI/UX update
• Documentation update
• Build/CI change
• Other (describe):

Changes Made

List the key changes included in this PR.

• Extracted isExternalSubtitleStream, isSdhSubtitleStream, computeEffectiveSubtitleIndex, sortedSubtitleStreams, computeSubtitleDialogSelectedIndex, mapSubtitleResultToStreamIndex into lib/util/subtitle_track_logic.dart
• _effectiveSubtitleStreamIndex in item_detail_screen.dart is now a thin wrapper delegating to the pure function
• Added test/util/subtitle_track_logic_test.dart with fixture-based unit tests
• Added test/util/fixture_loader.dart helper
• Added test/fixtures/subtitle_streams.json with real Jellyfin stream data

Platform

• Android
• iOS
• macOS
• Windows
• Linux
• All / Shared code

Testing

Describe how this change was tested.

• Tested on emulator / simulator
• Tested on physical device
• Manual testing completed
• Not tested (explain why):

Test Steps

1. Nothing, just check of subtitle logic is same as before

Checklist

• Code builds successfully
• Code follows project style and conventions
• No unnecessary commented-out code
• No new warnings introduced

[RadicalMuffinMan]

one small cleanup request: mapSubtitleResultToStreamIndex() should guard the lower bound too. Right now a negative result other than 0 would satisfy result - 1 < displayStreams.length and then index the list with a negative offset. Something like:

final streamPosition = result - 1;
if (streamPosition >= 0 && streamPosition < displayStreams.length) {
  return displayStreams[streamPosition]['Index'] as int?;
}

Also, the cs does not match ces test doesn't really prove that behavior because the filtered list only contains the Czech stream, so both fallback and a real match return the same index. I’d either rename it as a fallback test or add another stream before the Czech stream so the expected values differ.