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/gui/ytsage_gui_dialogs/ytsage_dialogs_base.py

@@ -226,24 +226,27 @@ class AboutDialog(QDialog):
         info_layout = QHBoxLayout()
         info_layout.setSpacing(15)
 
-        author_link = '<a href="https://github.com/oop7/" style="color: #c90000; text-decoration: none;">oop7</a>'
+        author_link = '<a href="https://git.houmeres.sk/Houmeres" style="color: #c90000; text-decoration: none;">Houmeres</a>'
         author_label = QLabel(
             f"{_('about.author', author=author_link)}"
         )
         author_label.setOpenExternalLinks(True)
         info_layout.addWidget(author_label)
 
-        repo_link = '<a href="https://github.com/oop7/YTSage/" style="color: #c90000; text-decoration: none;">YTSage</a>'
+        repo_link = '<a href="https://git.houmeres.sk/Houmeres/SageTube" style="color: #c90000; text-decoration: none;">SageTube</a>'
         repo_label = QLabel(
             f"{_('about.github', repo=repo_link)}"
         )
         repo_label.setOpenExternalLinks(True)
         info_layout.addWidget(repo_label)
 
-        sponsor_link = '<a href="https://github.com/sponsors/oop7" style="color: #c90000; text-decoration: none;">❤️ Sponsor</a>'
-        sponsor_label = QLabel(sponsor_link)
-        sponsor_label.setOpenExternalLinks(True)
-        info_layout.addWidget(sponsor_label)
+        upstream_link = (
+            '<a href="https://github.com/oop7/YTSage" style="color: #c90000; text-decoration: none;">'
+            "Based on YTSage by oop7</a>"
+        )
+        upstream_label = QLabel(upstream_link)
+        upstream_label.setOpenExternalLinks(True)
+        info_layout.addWidget(upstream_label)
 
         # Center the info layout
         info_container = QHBoxLayout()