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/assets/album.svg

@@ -0,0 +1,8 @@
+<svg viewBox="0 0 42 42" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g id="defaultAlbumImage">
+<g id="defaultAlbumImage_2">
+<path id="Vector" d="M21 31.5C26.799 31.5 31.5 26.799 31.5 21C31.5 15.201 26.799 10.5 21 10.5C15.201 10.5 10.5 15.201 10.5 21C10.5 26.799 15.201 31.5 21 31.5Z" stroke="#78777F" stroke-miterlimit="10" stroke-linecap="round" stroke-linejoin="round"/>
+<path id="Vector_2" d="M21 23.5C22.3807 23.5 23.5 22.3807 23.5 21C23.5 19.6193 22.3807 18.5 21 18.5C19.6193 18.5 18.5 19.6193 18.5 21C18.5 22.3807 19.6193 23.5 21 23.5Z" stroke="#78777F" stroke-miterlimit="10" stroke-linecap="round" stroke-linejoin="round"/>
+</g>
+</g>
+</svg>

src/swingmusic/assets/playlist.svg

@@ -0,0 +1,13 @@
+<svg viewBox="0 0 42 42" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g id="defaultPlaylistImage">
+<g id="defaultPlaylistImage_2">
+<g id="Group">
+<path id="Vector" d="M14.1 29.3C15.6464 29.3 16.9 28.0464 16.9 26.5C16.9 24.9536 15.6464 23.7 14.1 23.7C12.5536 23.7 11.3 24.9536 11.3 26.5C11.3 28.0464 12.5536 29.3 14.1 29.3Z" stroke="#78777F" stroke-miterlimit="10" stroke-linecap="round"/>
+<path id="Vector_2" d="M16.9 26.5V12.8" stroke="#78777F" stroke-miterlimit="10"/>
+<path id="Vector_3" d="M21 24.2H29.3" stroke="#78777F" stroke-miterlimit="10"/>
+<path id="Vector_4" d="M21 16.9H31.1" stroke="#78777F" stroke-miterlimit="10"/>
+<path id="Vector_5" d="M21 20.5H30.2" stroke="#78777F" stroke-miterlimit="10"/>
+</g>
+</g>
+</g>
+</svg>