fix: focus first unwatched episode in details view (#117)

[ProdigyV21]

Summary

Closes #117.

In the episode list on the details screen, the first unwatched episode is now focused by default instead of always focusing episode 1. Particularly valuable for long-running shows/anime where users had to scroll past everything they had already watched.

Root cause

DetailsViewModel.loadDetails() computes nextUnwatchedEpisode on line 496 and uses it for the play button label ("Continue S1E3"), but initialEpisodeIndex at line 493-495 always defaulted to 0 when the caller did not pass an explicit episode target (which is the normal case for navigation from the home grid, search, catalogs, etc. — only Continue Watching tiles pass a target episode).

Fix

Replaced the if/else 0 with a three-way when:

val initialEpisodeIndex = when {
    targetEpisodeForRow != null ->
        decoratedEpisodes.indexOfFirst { it.episodeNumber == targetEpisodeForRow }.coerceAtLeast(0)
    nextUnwatchedEpisode != null ->
        decoratedEpisodes.indexOf(nextUnwatchedEpisode).coerceAtLeast(0)
    else -> 0
}

The watched-status decoration already runs a few lines earlier (pulls from Trakt for linked users via traktRepository.getWatchedEpisodesForShow(mediaId), falls back to local season progress otherwise), so this reuses existing data with zero extra network calls or work.

Edge cases handled

• Explicit target from Continue Watching: unchanged behavior — the user still lands on the exact episode they were watching.
• Fully watched season: nextUnwatchedEpisode is null, falls back to index 0 (episode 1). User can use the season selector to jump forward.
• Brand new show (nothing watched): nextUnwatchedEpisode is episode 1, indexOf(...) returns 0, same as the old default.
• Switching seasons on an already-loaded show: the LaunchedEffect in DetailsScreen.kt:268 keys on (initialEpisodeIndex, episodes) — both change when a new season loads, so the new focus index is picked up correctly.
• Index-0 re-sync guard: DetailsScreen.kt:269 only re-syncs when initialEpisodeIndex > 0. When my fix returns 0, the screen-level episodeIndex default is also 0, so no re-sync is needed. No behavior change for shows where episode 1 is unwatched.

Risk

Minimal. Single-file, 13-line change. Reuses existing watched-episode data. No new network calls, no new allocations on a hot path.