Reorganize repository structure for better organization

- Move backend code to swingmusic/ folder
- Move client applications to root level (swingmusic-android, swingmusic-desktop, swingmusic-webclient)
- Remove intermediate backend/ and clients/ folders
- Update README with new folder structure and setup instructions
- Clean and organized repository layout

src/swingmusic/models/folder.py

@@ -1,9 +0,0 @@
-from dataclasses import dataclass
-
-
-@dataclass(slots=True, frozen=True)
-class Folder:
-    name: str
-    path: str
-    is_sym: bool = False
-    trackcount: int = 0