Move cover cache and temp files from /tmp to XDG cache (~/.cache/neo-pod-desktop/)

- Add xdg_cache_path() utility respecting
- CoverCache now stores covers in ~/.cache/neo-pod-desktop/covers/
- MetadataHandler temp files in ~/.cache/neo-pod-desktop/thumbnails/
- Survives reboot, no more lost cover_path during process_file

src/artwork/cache.py

@@ -9,21 +9,33 @@ from __future__ import annotations
 import hashlib
 import logging
 import os
-import tempfile
 
 logger = logging.getLogger(__name__)
 
 
+def xdg_cache_path(subdir: str) -> str:
+    """Return path inside XDG cache directory for neo-pod-desktop.
+    Creates the directory structure if missing.
+    """
+    base = os.environ.get(
+        "XDG_CACHE_HOME",
+        os.path.join(os.path.expanduser("~"), ".cache"),
+    )
+    path = os.path.join(base, "neo-pod-desktop", subdir)
+    os.makedirs(path, exist_ok=True)
+    return path
+
+
 class CoverCache:
     """Manages a local cache of cover art images for the player UI.
 
     The cache stores JPEG thumbnails keyed by (artist, album) hash.
+    Covers are stored in $XDG_CACHE_HOME/neo-pod-desktop/covers/
+    (default ~/.cache/neo-pod-desktop/covers/).
     """
 
     def __init__(self, cache_dir: str | None = None):
-        self.cache_dir = cache_dir or os.path.join(
-            tempfile.gettempdir(), "neo-pod-covers"
-        )
+        self.cache_dir = cache_dir or xdg_cache_path("covers")
         os.makedirs(self.cache_dir, exist_ok=True)
 
     def _key(self, artist: str, album: str) -> str: