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/utils/ytsage_config_manager.py

@@ -86,7 +86,7 @@ class ConfigManager:
         "geo_proxy_url": None,
         "auto_update_ytdlp": True,
         "auto_update_frequency": "daily",
-        "check_app_updates": True,
+        "check_app_updates": False,  # fork updates come from Gitea, not the upstream PyPI package
         "check_beta_updates": False,
         "last_update_check": 0,
         "concurrent_fragments": 1,