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/languages/en.json

@@ -22,7 +22,7 @@
     "restart_notice": "Language change will take effect after restarting the application."
   },
   "app": {
-    "title": "YTSage",
+    "title": "SageTube",
     "version": "v{version}",
     "ready": "Ready"
   },
@@ -240,9 +240,9 @@
     "update_in_progress_message": "yt-dlp is currently updating. Please wait a moment."
   },
   "about": {
-    "title": "About YTSage",
+    "title": "About SageTube",
     "version": "Version {version}",
-    "description": "Modern YouTube downloader with clean PySide6 interface.",
+    "description": "Watch-first YouTube client with embedded mpv playback and yt-dlp downloading. Fork of YTSage by oop7 (MIT).",
     "author": "By: {author}",
     "github": "GitHub: {repo}",
     "system_info": "System Information",