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>

pyproject.toml

@@ -3,11 +3,12 @@ requires = ["setuptools"]
 build-backend = "setuptools.build_meta"
 
 [project]
-name = "ytsage"
-version = "5.2.0"
-description = "Modern YouTube downloader with a clean PySide6 interface."
+name = "sagetube"
+version = "0.1.0"
+description = "Watch-first YouTube client with embedded mpv playback and yt-dlp downloading. Fork of YTSage."
 authors = [
-    { name = "oop7", email = "oop7_support@proton.me" },
+    { name = "Houmeres", email = "admin@ecoposta.sk" },
+    { name = "oop7 (YTSage upstream)", email = "oop7_support@proton.me" },
 ]
 dependencies = [
     "PySide6>=6.10.1",
@@ -48,12 +49,13 @@ keywords = ["youtube", "downloader", "video", "audio", "PySide6", "yt-dlp", "GUI
 
 
 [project.scripts]
+sagetube = "ytsage.main:main"
 ytsage = "ytsage.main:main"
 
 [project.urls]
-Homepage = "https://github.com/oop7/YTSage"
-Bug-Tracker = "https://github.com/oop7/YTSage/issues"
-Reddit = "https://www.reddit.com/r/NO-N_A_M_E/"
+Homepage = "https://git.houmeres.sk/Houmeres/SageTube"
+Bug-Tracker = "https://git.houmeres.sk/Houmeres/SageTube/issues"
+Upstream = "https://github.com/oop7/YTSage"
 
 [tool.setuptools]
 include-package-data = true