Check for SageTube updates, not YTSage's

The update check asked PyPI for the `ytsage` package's version, asked GitHub
for oop7/YTSage's release notes, and pointed the download button at upstream's
releases. None of that describes this program. Being reminded to install
YTSage was the visible symptom; the cause was that the check had never been
repointed when the fork was made.

It now reads SageTube's own releases from git.houmeres.sk. Gitea's release API
is shaped like GitHub's, so the dialog and the caller are unchanged -- the
class keeps its name and signal signature, and only its body moved out to
core/ytsage_app_update.py, which is fork-owned and will not conflict on the
next merge from upstream.

Upstream's inherited tags end in `b` (v5.3.0b and earlier). packaging reads
that as a beta marker, so they sort below v5.4.0 and a stable instance cannot
be handed one. Tag parsing is defensive anyway: one unparseable tag must not
take the whole check down with it.

Also: the check is rate-limited to once a day rather than every start, the
dialog gained a "Skip this version" that survives a restart, the thread is now
joined on close, and the About dialog says SageTube. The three binary updaters
-- yt-dlp, Deno, ffmpeg -- legitimately track their own upstreams and are
deliberately untouched; ytsage_app_update's docstring says so, because "update"
is an overloaded word in this codebase.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

ytsage/utils/ytsage_config_manager.py

@@ -92,6 +92,7 @@ class ConfigManager:
         "check_app_updates": False,  # fork updates come from Gitea, not the upstream PyPI package
         "check_beta_updates": False,
         "last_update_check": 0,
+        "skipped_update_version": None,  # set by the update dialog's "Skip this version"
         "concurrent_fragments": 1,
         "language": "en",
         "ytdlp_channel": "stable",