Move backend files to root level for cleaner GitHub display

- Move all backend files from swingmusic/ to root level
- Backend files now display directly on GitHub repository page
- Keep client applications as submodules (swingmusic-android, swingmusic-desktop, swingmusic-webclient)
- Update README to reflect new structure (no cd swingmusic needed)
- Cleaner, more professional GitHub repository layout

Files moved to root:
- src/ (main source code)
- pyproject.toml, requirements.txt, run.py
- swingmusic.spec, uv.lock, version.txt
- services/

Result: GitHub shows backend files directly while maintaining organized structure

src/swingmusic/lib/sortlib.py

@@ -0,0 +1,55 @@
+from itertools import groupby
+import os
+from typing import Callable
+from swingmusic.lib.albumslib import sort_by_track_no
+from swingmusic.models.folder import Folder
+from swingmusic.models.track import Track
+from swingmusic.utils import flatten
+
+
+def sort_tracks(tracks: list[Track], key: str, reverse: bool = False):
+    """
+    Sorts a list of tracks by a key.
+    """
+    if key == "default":
+        return tracks
+
+    sortfunc: Callable[[Track], str] = lambda track: getattr(track, key)
+    if key == "artists" or key == "albumartists":
+        sortfunc = lambda track: getattr(track, key)[0]["name"]
+
+    if key == "disc":
+        # INFO: Group tracks into albums, then sort them by disc number.
+        tracks = sorted(tracks, key=lambda x: x.album.casefold())
+        groups = groupby(tracks, lambda x: x.albumhash)
+
+        return flatten([sort_by_track_no(list(g)) for k, g in groups])
+
+    # INFO: sort tracks by title for a fallback value
+    tracks = sorted(tracks, key=lambda t: t.title.casefold())
+
+    if key == "title" and not reverse:
+        return tracks
+
+    return sorted(
+        tracks,
+        key=lambda track: sortfunc(track).casefold()
+        if isinstance(sortfunc(track), str)
+        else sortfunc(track),
+        reverse=reverse,
+    )
+
+
+def sort_folders(folders: list[Folder], key: str, reverse: bool = False):
+    """
+    Sorts a list of folders by a key.
+    """
+    if key == "default":
+        return folders
+
+    sortfunc: Callable[[Folder], str | float] = lambda folder: getattr(folder, key)
+
+    if key == "lastmod":
+        sortfunc = lambda folder: os.path.getmtime(folder.path)
+
+    return sorted(folders, key=sortfunc, reverse=reverse)