fix(sync): write playlist .m3u manifests to the device

[lqdev]

Summary

Closes #266 — Playlists copied to a device contained the audio files but no .m3u manifest, so media players couldn't open a synced playlist as a unit.

The local manifests were generated correctly (#255), but the device-sync path never copied them: scan_playlists_for_sync (src/download/manager.rs) only scanned each playlist's audio/ subfolder, so the sibling <name>.m3u was never added to the sync set. A verbatim copy would also have been broken — device sync flattens playlists (Playlists/<name>/audio/<file> → Playlists/<name>/<file>, no audio/ subfolder) while the local manifest references audio/<file>.

sync_to_device now writes a device-layout manifest at Playlists/<name>/<name>.m3u for every playlist with synced audio, rewriting each path line to drop the audio/ prefix while preserving #EXTM3U/#EXTINF titles and episode order.

Changes

• src/playlist/file_manager.rs: add pure PlaylistFileManager::transform_m3u_for_device helper (strips a leading audio/ or audio\ from path lines; leaves #EXTM3U/#EXTINF and already-flat paths untouched).
• src/download/manager.rs: add a sync_playlist_manifests step to sync_to_device (after audio copy, before orphan deletion). It:
  • derives playlists-with-synced-audio from pc_files,
  • writes the transformed manifest to the device,
  • is idempotent (reads the device file directly and skips unchanged manifests — .m3u is excluded from the audio-only device_files scan),
  • honors dry_run (reported, not written),
  • is regenerated by hard_sync,
  • runs its own orphan pass to delete a stale device manifest once its playlist no longer has synced audio (the main orphan loop never sees .m3u).
• CHANGELOG.md: [Unreleased] → Fixed entry.
• docs/DEVICE_PROFILES.md: correct the playlist-manifest paragraph to describe the flat device layout.

Manual Testing

1. Add episodes to a playlist (so …/data/playlists/<name>/<name>.m3u exists locally).
2. Open the Sync buffer (F8), pick the device directory, and run a sync.
3. Before: <device>/Playlists/<name>/ held only 001-….mp3 files, no manifest.
   After: <device>/Playlists/<name>/<name>.m3u exists; open it in a media player → loads the episodes in order.
4. Inspect the manifest: entries are flat (001-….mp3), not audio/001-….mp3; #EXTINF titles preserved.
5. Re-run the sync → manifest is skipped (not rewritten/duplicated).
6. Delete the playlist, sync again → its device .m3u (and audio) are removed.

Tests

• 5 unit tests for the transform helper (file_manager.rs): strip prefix, preserve directives/order, header-only, non-audio/ passthrough, backslash prefix.
• 5 integration tests (manager.rs): device m3u written with stripped paths, idempotent skip + not orphan-deleted, removed when playlist removed, dry-run writes nothing but reports, hard-sync regenerates.
• Full suite green (718 lib + integration tests).

Quality

• cargo fmt --check ✅
• cargo clippy --all-targets -- -D warnings ✅
• cargo test ✅

[Copilot]

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