Store track paths relative to the data dir for multi-machine sync

The library is going live as the canonical store, synced to a second machine
via Syncthing. Track locations were stored absolute and only remapped at import,
so on a second machine (where Syncthing mounts the folder at a different path)
every location would break.

- lintunes/paths.py: to_relative/to_absolute. Locations are stored relative to
  the data dir and resolved back on load, applied only at the json_storage
  boundary (save_tracks/load_library). Track.location stays absolute in memory,
  so the player, tagging, and art code are unchanged. The data dir and the music
  move together inside one synced tree, so paths resolve wherever it's mounted —
  no per-machine music_root config. Absolute paths in older library.json files
  still load (back-compat).
- tests/test_round15.py: helper round-trips, back-compat, and a machine-2
  scenario (save under root A, load the copied tree under root B).
- TASKS.md/tasks-done.md: mark the data-dir move + real import done; log the
  benign exit-time Qt/FFmpeg teardown segfault.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lintunes/storage/json_storage.py

@@ -2,6 +2,7 @@ import json
 from pathlib import Path
 
 from lintunes.models import Track, Playlist, PlaylistSettings, Library
+from lintunes.paths import to_relative, to_absolute
 
 
 def save_library(library: Library, data_dir: Path):
@@ -23,7 +24,14 @@ def save_library(library: Library, data_dir: Path):
 
 def save_tracks(library: Library, data_dir: Path):
     data_dir.mkdir(parents=True, exist_ok=True)
-    tracks_dict = {str(tid): track.to_dict() for tid, track in library.tracks.items()}
+    tracks_dict = {}
+    for tid, track in library.tracks.items():
+        data = track.to_dict()
+        if "location" in data:
+            # Store paths relative to the data dir so the library is portable
+            # across machines that sync the folder to different mount points.
+            data["location"] = to_relative(data["location"], data_dir)
+        tracks_dict[str(tid)] = data
     _write_json(data_dir / "library.json", tracks_dict)
 
 
@@ -62,6 +70,8 @@ def load_library(data_dir: Path) -> Library:
         tracks_dict = _read_json(library_path)
         for tid_str, track_data in tracks_dict.items():
             track = Track.from_dict(track_data)
+            # Stored relative to the data dir; resolve back to an absolute path.
+            track.location = to_absolute(track.location, data_dir)
             tracks[track.track_id] = track
 
     # Load metadata