Fix track duration shrinking when seeking near the end with smart crossfade

[MarvinSchenkel]

What does this implement/fix?

With smart crossfade enabled, the last part of each track (up to 45s) is held back as the crossfade tail. The end-of-track duration recalculation excluded this tail, so the reported track duration shrank by up to the crossfade length. After seeking near the end of a track the error is large enough that the duration drops below the play position, breaking the seekbar and the duration/position reported to HA entities. The shrunken value was also reused on the next seek, compounding the error.

• Flow mode: include the held-back crossfade tail when recalculating the track duration, and drop the now duplicate end-of-queue correction
• Single item mode: credit the full fade-out tail (instead of the trimmed fadeout share of the mix) to the duration

Related issue (if applicable):

• related issue Reported media duration suddenly decreases if position is near the end of a track + more support#5494

Types of changes

• Bugfix (non-breaking change which fixes an issue) — bugfix
• New feature (non-breaking change which adds functionality) — new-feature
• Enhancement to an existing feature — enhancement
• New music/player/metadata/plugin provider — new-provider
• Breaking change (fix or feature that would cause existing functionality to not work as expected) — breaking-change
• Refactor (no behaviour change) — refactor
• Documentation only — documentation
• Maintenance / chore — maintenance
• CI / workflow change — ci
• Dependencies bump — dependencies

Checklist

• The code change is tested and works locally.
• pre-commit run --all-files passes.
• pytest passes, and tests have been added/updated under tests/ where applicable.
• For changes to shared models, the companion PR in music-assistant/models is linked.
• For changes affecting the UI, the companion PR in music-assistant/frontend is linked.
• I have read and complied with the project's AI Policy for any AI-assisted contributions.
• I have raised a PR against the documentation repository targeting the main or beta branch as appropriate.

[Copilot]

Pull request overview

This PR fixes incorrect track duration recalculation when smart crossfade is enabled, specifically preventing the reported duration from shrinking after seeking near the end of a track (which could break UI seekbars and HA entity state).

Changes:

• Single-item (gapless) smartfade: account for the held-back fade-out tail when recomputing streamdetails.duration.
• Flow-mode streaming: include the held-back crossfade tail in per-track duration recomputation, and remove the now-duplicate end-of-queue duration correction.