Rebrand user-visible surfaces to SageTube

The internal ytsage package name is kept deliberately - renaming it
would touch every file and destroy the ability to merge upstream
YTSage changes.

- pyproject: distribution name sagetube v0.1.0, sagetube entrypoint
  (ytsage alias retained), URLs point at the Gitea repo with an
  Upstream link to YTSage.
- Data dirs move to SageTube/ (fresh fork, fresh state) and the config
  file becomes sagetube_config.json; QApplication name and window
  title read SageTube.
- About dialog credits Houmeres and links "Based on YTSage by oop7".
- App self-update check against the upstream PyPI package is disabled
  by default; yt-dlp/deno/ffmpeg update flows are unaffected.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ytsage/main.py

@@ -25,6 +25,8 @@ def main():
     try:
         logger.info("Starting YTSage application")
         app = QApplication(sys.argv)
+        app.setApplicationName("SageTube")
+        app.setDesktopFileName("sagetube")
 
         window = YTSageApp()  # Instantiate the main application class
         window.show()